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MSTEST0081: '[TestFilterProvider]' should reference a valid test filter type

Property Value
Rule ID MSTEST0081
Title '[TestFilterProvider]' should reference a valid test filter type
Category Usage
Fix is breaking or non-breaking Non-breaking
Enabled by default Yes
Default severity Warning
Introduced in version 4.4.0 (preview)
Is there a code fix No

Important

This analyzer is planned for MSTest 4.4 and is available only in preview builds until MSTest 4.4.0 is released.

Cause

[assembly: TestFilterProvider(typeof(MyFilter))] references a type that doesn't satisfy the requirements the adapter enforces at run time, or the assembly registers more than one test filter provider.

Rule description

[assembly: TestFilterProvider(typeof(MyFilter))] passes the filter type as a Type, so the compiler accepts any type at all. The adapter validates the type only when it materializes the filter, and then fails the whole run. This analyzer reports the same problems at build time, where they're cheap to fix. The referenced type must:

  • Be non-generic.
  • Be instantiable, so it can't be abstract, static, an interface, or a byref-like type.
  • Implement ITestFilter.
  • Declare a public parameterless constructor (every struct already satisfies this).

At most one test filter provider can be registered per test assembly, and passing an explicit null filter type also fails. When targeting .NET, the generic [assembly: TestFilterProvider<MyFilter>] form enforces the interface and constructor requirements through generic constraints instead, so only the generic-type and "at most one provider" checks still apply to it.

[assembly: TestFilterProvider(typeof(MyFilter))]
public sealed class MyFilter : ITestFilter
{
    public MyFilter(string mode) { } // Violation: no public parameterless constructor
    public TestFilterResult Filter(TestFilterContext context) => TestFilterResult.Run;
}

How to fix violations

Add a public parameterless constructor, or otherwise adjust the type so that it's non-generic, instantiable, implements ITestFilter, and has a public parameterless constructor.

public sealed class MyFilter : ITestFilter
{
    public TestFilterResult Filter(TestFilterContext context) => TestFilterResult.Run;
}

When to suppress warnings

Don't suppress warnings from this rule. A violation fails the whole test run at run time rather than only the filter itself.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable MSTEST0081
// The code that's violating the rule is on this line.
#pragma warning restore MSTEST0081

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.MSTEST0081.severity = none

For more information, see How to suppress code analysis warnings.

See also

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