Record is immutable at compile time but it's not at runtime.

[moh-hassan]

Using Net 5 Rc1 and Creating a record:

public record Rectangle
{
    public int Width { get; init; }
    public int Height { get; init; }
}

Then instantiating it:

var rectangle = new Rectangle (20,30);

Trying to change the value:

rectange.Width=50; //compiler error

Compiler raise the error:

error CS8852: Init-only property or indexer 'Rectangle.Width' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

That is right and insure that the record is immutable.

Using a method like to test IsImmutable type give false, because in record there is no generated readonly properties.

How to check the record in c# 9 is immutable at runtime or even it's record with init property ?

[YairHalberstadt]

A record is indeed mutable at runtime. This is intentional, is it means most serializer frameworks work without updating.

It is however possible to check if a property is init by checking:

bool IsInitOnly(PropertyInfo propertyInfo)
{
    return propertyInfo.SetMethod.ReturnParameter.GetRequiredCustomModifiers().Any(x => x.FullName == typeof(System.Runtime.CompilerServices.IsExternalInit).FullName);
}

[MgSam]

They should put a convenience property in the BCL that checks this.

[moh-hassan]

Thanks @YairHalberstadt for answer.
It returns false for both fields.

[YairHalberstadt]

Sorry I made a mistake - updated my above answer

[moh-hassan]

Now, it returns true :) for both properties.
So, I'll check all properties of record returns true.
just a question:
I want to apply this method to records only,
how to check the type is record to selectively apply this method at runtime (reflection show record as a class)?

[YairHalberstadt]

There's no official way to check if a type is a record, however you can check if it has a private method called <Clone>$(). Since this is an illegal name in C#, it is almost definitely auto generated as part of a record.

[333fred]

It is for now. There is no guarantee that this will remain true, and there is intentionally no way to determine whether a type is a record or not.

[moh-hassan]

Thank you @YairHalberstadt for help and support.
It's working.

[moh-hassan]

@333fred
Ok, I'll take your note into consideration.