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xUnit1040 Warning

The type argument to theory data is nullable, while the type of the corresponding test method parameter is not

Cause

The TheoryData or TheoryDataRow type argument is marked as nullable, and the test method argument is marked as non-nullable.

Reason for rule

Passing null data to a test method that isn’t expecting it could cause runtime errors or unpredictable test results (either false positives or false negatives).

How to fix violations

To fix a violation of this rule, either make the theory data type non-nullable, or make the test method parameter nullable.

Examples

Violates

Using TheoryData<> (for v2 and v3)

using Xunit;

public class xUnit1040
{
    public static TheoryData<string?> PropertyData =>
        new() { "Hello", "World", default(string) };

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string _) { }
}

Using TheoryDataRow<> (for v3 only)

using System.Collections.Generic;
using Xunit;

public class xUnit1040
{
    public static IEnumerable<TheoryDataRow<string?>> PropertyData =>
        [new("Hello"), new("World"), new(null)];

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string _) { }
}

using System.Collections;
using System.Collections.Generic;
using Xunit;

public class ClassRowData : IEnumerable<TheoryDataRow<string?>>
{
    public IEnumerator<TheoryDataRow<string?>> GetEnumerator()
    {
        yield return new("Hello");
        yield return new("World");
        yield return new(null);
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public class xUnit1040
{
    [Theory]
    [ClassData(typeof(ClassRowData))]
    public void TestMethod(string _) { }
}

Does not violate

Using TheoryData<> (for v2 and v3)

using Xunit;

public class xUnit1040
{
    public static TheoryData<string> PropertyData =>
        new() { "Hello", "World" };

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string _) { }
}

using Xunit;

public class xUnit1040
{
    public static TheoryData<string?> PropertyData =>
        new() { "Hello", "World", default(string) };

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string? _) { }
}

Using TheoryDataRow<> (for v3 only)

using System.Collections.Generic;
using Xunit;

public class xUnit1040
{
    public static IEnumerable<TheoryDataRow<string>> PropertyData =>
        [new("Hello"), new("World")];

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string _) { }
}

using System.Collections.Generic;
using Xunit;

public class xUnit1040
{
    public static IEnumerable<TheoryDataRow<string?>> PropertyData =>
        [new("Hello"), new("World"), new(null)];

    [Theory]
    [MemberData(nameof(PropertyData))]
    public void TestMethod(string? _) { }
}

using System.Collections;
using System.Collections.Generic;
using Xunit;

public class ClassRowData : IEnumerable<TheoryDataRow<string>>
{
    public IEnumerator<TheoryDataRow<string>> GetEnumerator()
    {
        yield return new("Hello");
        yield return new("World");
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public class xUnit1040
{
    [Theory]
    [ClassData(typeof(ClassRowData))]
    public void TestMethod(string _) { }
}

using System.Collections;
using System.Collections.Generic;
using Xunit;

public class ClassRowData : IEnumerable<TheoryDataRow<string?>>
{
    public IEnumerator<TheoryDataRow<string?>> GetEnumerator()
    {
        yield return new("Hello");
        yield return new("World");
        yield return new(null);
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public class xUnit1040
{
    [Theory]
    [ClassData(typeof(ClassRowData))]
    public void TestMethod(string? _) { }
}
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