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Edit pass to combine different concepts
Combine errors on different concepts related to the `params` modifier.
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‎docs/csharp/language-reference/compiler-messages/params-arrays.md‎

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@@ -50,266 +50,87 @@ There are a few *errors* related to the `lock` statement and thread synchronizat
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<!-- The text in this list generates issues for Acrolinx, because they don't use contractions.
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That's by design. The text closely matches the text of the compiler error / warning for SEO purposes.
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- **CS0225**: *The params parameter must be a single-dimensional array or have a valid collection type*
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- **CS0231**: *A params parameter must be the last parameter in a formal parameter list.*
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- **CS0466**: *'method1' should not have a params parameter since 'method2' does not*
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- **CS0674**: *Do not use `System.ParamArrayAttribute` or `System.ParamArrayAttribute`/`System.Runtime.CompilerServices.ParamCollectionAttribute`. Use the `params` keyword instead.*
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- **CS0758**: *Both partial method declarations must use a `params` parameter or neither may use a `params` parameter*
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- **CS1104**: *A parameter array cannot be used with `this` modifier on an extension method.*
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- **CS1611**: *The params parameter cannot be declared as in `ref` or `out`*
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- **CS1670**: *`params` is not valid in this context*
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- **CS1751**: *Cannot specify a default value for a parameter array.*
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- **CS9218**: *The type arguments for method cannot be inferred from the usage because an argument with dynamic type is used and the method has a non-array params collection parameter. Try specifying the type arguments explicitly.*
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- **CS9219**: *Ambiguity between expanded and normal forms of non-array params collection parameter of, the only corresponding argument has the type 'dynamic'. Consider casting the dynamic argument.*
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- **CS9223**: *Creation of params collection results in an infinite chain of invocation of constructor.*
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- **CS9224**: *Method cannot be less visible than the member with params collection.*
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- **CS9225**: *Constructor leaves required member uninitialized.*
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- **CS9227**: *Type does not contain a definition for a suitable instance `Add` method.*
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- **CS9228**: *Non-array params collection type must have an applicable constructor that can be called with no arguments.*
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- [**CS0225**](#parameter-and-argument-type-rules): *The params parameter must be a single-dimensional array or have a valid collection type*
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- [**CS0231**](#method-declaration-rules): *A params parameter must be the last parameter in a formal parameter list.*
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- [**CS0466**](#other-params-errors): *'method1' should not have a params parameter since 'method2' does not*
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- [**CS0674**](#other-params-errors): *Do not use `System.ParamArrayAttribute` or `System.ParamArrayAttribute`/`System.Runtime.CompilerServices.ParamCollectionAttribute`. Use the `params` keyword instead.*
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- [**CS0758**](#other-params-errors): *Both partial method declarations must use a `params` parameter or neither may use a `params` parameter*
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- [**CS1104**](#method-declaration-rules): *A parameter array cannot be used with `this` modifier on an extension method.*
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- [**CS1611**](#method-declaration-rules): *The params parameter cannot be declared as in `ref` or `out`*
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- [**CS1670**](#method-declaration-rules): *`params` is not valid in this context*
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- [**CS1751**](#method-declaration-rules): *Cannot specify a default value for a parameter array.*
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- [**CS9218**](#parameter-and-argument-type-rules): *The type arguments for method cannot be inferred from the usage because an argument with dynamic type is used and the method has a non-array params collection parameter. Try specifying the type arguments explicitly.*
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- [**CS9219**](#parameter-and-argument-type-rules): *Ambiguity between expanded and normal forms of non-array params collection parameter of, the only corresponding argument has the type `dynamic`. Consider casting the dynamic argument.*
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- [**CS9223**](#other-params-errors): *Creation of params collection results in an infinite chain of invocation of constructor.*
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- [**CS9224**](#other-params-errors): *Method cannot be less visible than the member with params collection.*
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- [**CS9225**](#other-params-errors): *Constructor leaves required member uninitialized.*
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- [**CS9227**](#parameter-and-argument-type-rules): *Type does not contain a definition for a suitable instance `Add` method.*
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- [**CS9228**](#parameter-and-argument-type-rules): *Non-array params collection type must have an applicable constructor that can be called with no arguments.*
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In addition, the compiler might produce the following *warning* related to the `params` modifier on method parameters:
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- **CS9220**: *One or more overloads of method having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- **CS9221**: *One or more indexer overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- **CS9222**: *One or more constructor overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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## Compiler Error CS0225
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The params parameter must be a single-dimensional array
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The params parameter must have a valid collection type
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When using the [params](../keywords/method-parameters.md#params-modifier) keyword, you must specify a single-dimensional array or collection of the data type. For more information, see [Methods](../../programming-guide/classes-and-structs/methods.md).
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The following sample generates CS0225:
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```csharp
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// CS0225.cs
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public class MyClass
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{
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public static void TestParams(params int a) // CS0225
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// try the following line instead
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// public static void TestParams(params int[] a)
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{
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}
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public static void Main()
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{
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TestParams(1);
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}
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}
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```
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## Compiler Error CS0231
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A params parameter must be the last parameter in a formal parameter list.
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The [params](../keywords/method-parameters.md#params-modifier) parameter supports a variable number of arguments and must be after all other parameters. For more information, see [Methods](../../programming-guide/classes-and-structs/methods.md).
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The following sample generates CS0231:
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```csharp
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// CS0231.cs
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class Test
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{
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public void TestMethod(params int[] p, int i) {} // CS0231
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// To resolve the error, use the following line instead:
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// public void TestMethod(int i, params int[] p) {}
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static void Main()
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{
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}
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}
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```
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## Compiler Error CS0466
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'method1' should not have a params parameter since 'method2' does not
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You cannot use `params` parameter on a class member if the implemented interface doesn't use it.
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The following sample generates CS0466.
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```csharp
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// CS0466.cs
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interface I
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{
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void F1(params int[] a);
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void F2(int[] a);
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}
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class C : I
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{
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void I.F1(params int[] a) {}
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void I.F2(params int[] a) {} // CS0466
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void I.F2(int[] a) {} // OK
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public static void Main()
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{
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I i = (I) new C();
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i.F1(new int[] {1, 2} );
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i.F2(new int[] {1, 2} );
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}
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}
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```
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## Compiler Error CS0674
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Do not use 'System.ParamArrayAttribute'. Use the 'params' keyword instead.
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Do not use 'System.ParamArrayAttribute'/'System.Runtime.CompilerServices.ParamCollectionAttribute'. Use the 'params' keyword instead.
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- [**CS9220**](#parameter-and-argument-type-rules): *One or more overloads of method having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- [**CS9221**](#parameter-and-argument-type-rules): *One or more indexer overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- [**CS9222**](#parameter-and-argument-type-rules): *One or more constructor overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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The C# compiler does not allow for the use of <xref:System.ParamArrayAttribute?displayProperty=nameWithType>; use [params](../keywords/method-parameters.md#params-modifier) instead.
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## Method declaration rules
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The following sample generates CS0674:
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The following errors indicate using a `params` modifier on a parameter when the `params` modifier isn't allowed in that context:
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```csharp
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// CS0674.cs
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using System;
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public class MyClass
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{
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public static void UseParams([ParamArray] int[] list) // CS0674
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// try the following line instead
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// public static void UseParams(params int[] list)
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{
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for ( int i = 0 ; i < list.Length ; i++ )
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Console.WriteLine(list[i]);
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Console.WriteLine();
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}
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public static void Main()
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{
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UseParams(1, 2, 3);
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}
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}
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```
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## Compiler Error CS0758
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Both partial method declarations must use a params parameter or neither may use a params parameter
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If one part of a partial method specifies a `params` parameter, the other part must specify one also.
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Either add the `params` modifier in one part of the method, or remove it in the other.
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The following code generates CS0758:
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```csharp
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using System;
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public partial class C
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{
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partial void Part(int i, params char[] array);
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partial void Part(int i, char[] array) // CS0758
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{
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}
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public static int Main()
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{
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return 1;
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}
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}
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```
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## Compiler Error CS1104
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A parameter array cannot be used with 'this' modifier on an extension method.
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The first parameter of an extension method cannot be a params array.
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Remember that the first parameter of an extension method definition specifies which type the method will "extend". It is not an input parameter. Therefore, it makes no sense to have a params array in this location. If you do have to pass in a params array, make it the second parameter.
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The following example generates CS1104:
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```csharp
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// cs1104.cs
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// Compile with: /target:library
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public static class Extensions
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{
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public static void Test<T>(this params T[] tArr) {} // CS1104
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}
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```
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## Compiler Error CS1611
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The params parameter cannot be declared as in ref or out
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The keywords [in](../keywords/method-parameters.md#in-parameter-modifier), [ref](../keywords/ref.md) or [out](../keywords/method-parameters.md#out-parameter-modifier) cannot be used with the [params](../keywords/method-parameters.md#params-modifier) keyword.
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The following sample generates CS1611:
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```csharp
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// CS1611.cs
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public class MyClass
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{
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public static void Test(params ref int[] a) // CS1611, remove ref
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{
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}
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public static void Main()
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{
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Test(1);
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}
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}
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```
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- **CS0231**: *A params parameter must be the last parameter in a formal parameter list.*
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- **CS1104**: *A parameter array cannot be used with `this` modifier on an extension method.*
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- **CS1611**: *The params parameter cannot be declared as in `ref` or `out`*
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- **CS1670**: *`params` is not valid in this context*
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- **CS1751**: *Cannot specify a default value for a parameter array.*
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## Compiler Error CS1670
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The compiler enforces the following rules on your use of the `params` modifier on a method parameter:
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params is not valid in this context
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- The `params` modifier is allowed only on the last parameter in a formal parameter list. This includes any parameters with a default value.
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- You can't include a default argument for the parameter when the `params` modifier is used.
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- The `params` modifier can't be applied to reference parameter. A reference parameter is one with the `in`, `ref readonly`, `ref` or `out` modifier.
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- The `params` modifier can't be combined with the `this` modifier on an extension method.
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- The `params` modifier can't be used on an overloaded operator.
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A number of C# features are incompatible with variable argument lists, and do not allow the `params` keyword, including the following:
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In versions before C# 12, the `params` modifier can't be used on the parameter of an anonymous method or lambda expression.
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- Parameter lists of anonymous methods
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- Overloaded operators
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## Parameter and argument type rules
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The following sample generates CS1670:
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The following errors indicate that the type of the parameter used with `params` is invalid:
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```csharp
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// CS1670.cs
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public class C
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{
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public bool operator +(params int[] paramsList) // CS1670
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{
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return false;
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}
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- **CS9218**: *The type arguments for method cannot be inferred from the usage because an argument with dynamic type is used and the method has a non-array params collection parameter. Try specifying the type arguments explicitly.*
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- **CS9219**: *Ambiguity between expanded and normal forms of non-array params collection parameter of, the only corresponding argument has the type 'dynamic'. Consider casting the dynamic argument.*
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- **CS0225**: *The params parameter must be a single-dimensional array or have a valid collection type*
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- **CS9227**: *Type does not contain a definition for a suitable instance `Add` method.*
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- **CS9228**: *Non-array params collection type must have an applicable constructor that can be called with no arguments.*
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static void Main()
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{
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}
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}
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```
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The following warnings indicate that the one of the possible overloads might involve dynamic dispatch. The dynamic nature of the argument doesn't participate in overload resolution.
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## Compiler Error CS1751
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- **CS9220**: *One or more overloads of method having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- **CS9221**: *One or more indexer overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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- **CS9222**: *One or more constructor overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.*
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Cannot specify a default value for a parameter array.
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In versions before C# 13, the `params` modifier is allowed on single-dimensional arrays only. No other types were valid.
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The following sample generates CS1751:
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Starting with C# 13 any valid collection type can be used. However, some restrictions remain. The collection type must follow the same rules as the target of a [collection expression](../operators/collection-expressions.md#conversions).
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```csharp
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// CS1751.cs
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void Method(params object[] values = null)
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{
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}
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```
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## Other params errors
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To fix the error:
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The following errors indicate other issues with using the `params` modifier:
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```csharp
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// Explicitly passing null
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object[] values = null;
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Method(values);
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- **CS0466**: *'method1' should not have a params parameter since 'method2' does not*
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- **CS0674**: *Do not use `System.ParamArrayAttribute` or `System.Runtime.CompilerServices.ParamCollectionAttribute`. Use the `params` keyword instead.*
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- **CS0758**: *Both partial method declarations must use a `params` parameter or neither may use a `params` parameter*
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- **CS9223**: *Creation of params collection results in an infinite chain of invocation of constructor.*
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- **CS9224**: *Method cannot be less visible than the member with params collection.*
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- **CS9225**: *Constructor leaves required member uninitialized.*
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void Method(params object[] values)
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{
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if (values == null)
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{
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A method that implements an interface must include the `params` modifier if and only if the interface member has the `params` modifier. Similarly, either both declarations of a `partial` method must include the `params` modifier, or none can include the `params` modifier.
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}
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}
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```
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You must use the `params` modifier. You can't apply the equivalent attributes, either <xref:System.ParamArrayAttribute?displayProperty=nameWithType> or <xref:System.Runtime.CompilerServices.ParamCollectionAttribute?displayProperty=nameWithType>.
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## See also
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- [Extension Methods](../../programming-guide/classes-and-structs/extension-methods.md)
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- [Partial Classes and Methods](../../programming-guide/classes-and-structs/partial-classes-and-methods.md)
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- [Collection expressions](../operators/collection-expressions.md)
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- [params](../keywords/method-parameters.md#params-modifier)

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