diff --git a/CSharpExtender/Attributes/UniqueItemsAttribute.cs b/CSharpExtender/Attributes/UniqueItemsAttribute.cs
new file mode 100644
index 0000000..0a43cdb
--- /dev/null
+++ b/CSharpExtender/Attributes/UniqueItemsAttribute.cs
@@ -0,0 +1,91 @@
+using System;
+using System.Collections;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Reflection;
+
+namespace CSharpExtender.Attributes;
+
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
+public class UniqueItemsAttribute : ValidationAttribute
+{
+    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+    {
+        // If the value is null, let [Required] handle it if needed
+        if (value == null)
+        {
+            return ValidationResult.Success;
+        }
+
+        // Ensure the value is a collection.
+        // Need to explicitly check for string, as it is enumerable.
+        if (value is string || value is not IEnumerable collection)
+        {
+            return new ValidationResult("The UniqueItemsAttribute must be applied to a collection.");
+        }
+
+        var items = collection.Cast<object>().ToList();
+        if (items.Count == 0)
+        {
+            return ValidationResult.Success;
+        }
+
+        // Check for duplicates based on type
+        for (int i = 0; i < items.Count - 1; i++)
+        {
+            for (int j = i + 1; j < items.Count; j++)
+            {
+                if (AreItemsEqual(items[i], items[j]))
+                {
+                    // Use ErrorMessage if provided; otherwise, use default
+                    string errorMessage = ErrorMessage ??
+                        $"The list contains duplicate items at indices {i} and {j}.";
+                    return new ValidationResult(errorMessage);
+                }
+            }
+        }
+
+        return ValidationResult.Success;
+    }
+
+    private bool AreItemsEqual(object item1, object item2)
+    {
+        if (item1 == null && item2 == null)
+        {
+            return true;
+        }
+        if (item1 == null || item2 == null)
+        {
+            return false;
+        }
+
+        // Handle simple types (e.g., string, int)
+        if (item1.GetType().IsValueType || item1 is string)
+        {
+            return item1.Equals(item2);
+        }
+
+        // Handle complex objects by comparing all properties
+        var properties = item1.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);
+        foreach (var prop in properties)
+        {
+            var value1 = prop.GetValue(item1);
+            var value2 = prop.GetValue(item2);
+
+            if (value1 == null && value2 == null)
+            {
+                continue;
+            }
+            if (value1 == null || value2 == null)
+            {
+                return false;
+            }
+            if (!value1.Equals(value2))
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
\ No newline at end of file
diff --git a/README.md b/README.md
index ede55f5..488c5c0 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,12 @@
 
 Extension methods and classes I often find useful for C# development.
 
+## DataAnnotations
+
+Attribute classes to validate properties in models.
+
+- **`UniqueItemsAttribute`**: Check that a collection property does not contain duplicate items.
+
 ## Collections
 
 ### GenericCache
diff --git a/Test.CSharpExtender/Attributes/Test_UniqueItemsAttribute.cs b/Test.CSharpExtender/Attributes/Test_UniqueItemsAttribute.cs
new file mode 100644
index 0000000..3dd97f7
--- /dev/null
+++ b/Test.CSharpExtender/Attributes/Test_UniqueItemsAttribute.cs
@@ -0,0 +1,181 @@
+using CSharpExtender.Attributes;
+using System.ComponentModel.DataAnnotations;
+
+namespace Test.CSharpExtender.Attributes;
+
+public class UniqueItemsAttributeTests
+{
+    private static ValidationContext GetValidationContext(object instance) => new(instance);
+
+    // Test class for complex object validation
+    private class TestItem(int id, string name)
+    {
+        public int Id { get; set; } = id;
+        public string Name { get; set; } = name;
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_NullList_ReturnsSuccess()
+    {
+        var model = new { Items = null as List<string> };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.Equal(ValidationResult.Success, result);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_EmptyList_ReturnsSuccess()
+    {
+        var model = new { Items = new List<string>() };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.Equal(ValidationResult.Success, result);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_UniqueStrings_ReturnsSuccess()
+    {
+        var model = new { Items = new List<string> { "apple", "banana", "cherry" } };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.Equal(ValidationResult.Success, result);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateStrings_DefaultErrorMessage()
+    {
+        var model = new { Items = new List<string> { "apple", "apple", "cherry" } };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Contains("duplicate items", result.ErrorMessage);
+        Assert.Contains("indices 0 and 1", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateStrings_CustomErrorMessage()
+    {
+        var model = new { Items = new List<string> { "apple", "apple", "cherry" } };
+        var attribute = new UniqueItemsAttribute { ErrorMessage = "All items must be unique in the list." };
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Equal("All items must be unique in the list.", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_UniqueNumbers_ReturnsSuccess()
+    {
+        var model = new { Items = new List<int> { 1, 2, 3 } };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.Equal(ValidationResult.Success, result);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateNumbers_DefaultErrorMessage()
+    {
+        var model = new { Items = new List<int> { 1, 2, 2 } };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Contains("duplicate items", result.ErrorMessage);
+        Assert.Contains("indices 1 and 2", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateNumbers_CustomErrorMessage()
+    {
+        var model = new { Items = new List<int> { 1, 2, 2 } };
+        var attribute = new UniqueItemsAttribute { ErrorMessage = "Numbers must be unique." };
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Equal("Numbers must be unique.", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_UniqueComplexObjects_ReturnsSuccess()
+    {
+        var model = new
+        {
+            Items = new List<TestItem>
+            {
+                new TestItem(1, "apple"),
+                new TestItem(2, "banana")
+            }
+        };
+
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+        
+        Assert.Equal(ValidationResult.Success, result);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateComplexObjects_DefaultErrorMessage()
+    {
+        var model = new
+        {
+            Items = new List<TestItem>
+            {
+                new TestItem(1, "apple"),
+                new TestItem(1, "apple")
+            }
+        };
+
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+        
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Contains("duplicate items", result.ErrorMessage);
+        Assert.Contains("indices 0 and 1", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_DuplicateComplexObjects_CustomErrorMessage()
+    {
+        var model = new
+        {
+            Items = new List<TestItem>
+            {
+                new TestItem(1, "apple"),
+                new TestItem(1, "apple")
+            }
+        };
+
+        var attribute = new UniqueItemsAttribute { ErrorMessage = "Objects in the list must have unique values." };
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+        
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Equal("Objects in the list must have unique values.", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_SingleString_ReturnsError()
+    {
+        var model = new { Items = "not a list" };
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Contains("must be applied to a collection", result.ErrorMessage);
+    }
+
+    [Fact]
+    public void UniqueItemsAttribute_NonCollectionNonString_ReturnsError()
+    {
+        var model = new { Items = 42 }; // An int, not a string or collection
+        var attribute = new UniqueItemsAttribute();
+        var result = attribute.GetValidationResult(model.Items, GetValidationContext(model));
+
+        Assert.NotEqual(ValidationResult.Success, result);
+        Assert.Contains("must be applied to a collection", result.ErrorMessage);
+    }
+}
\ No newline at end of file
diff --git a/Test.CSharpExtender/Test.CSharpExtender.csproj b/Test.CSharpExtender/Test.CSharpExtender.csproj
index 09d3916..ee9e73d 100644
--- a/Test.CSharpExtender/Test.CSharpExtender.csproj
+++ b/Test.CSharpExtender/Test.CSharpExtender.csproj
@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>