MSTEST0064: Prefer async assertion methods

Property	Value
Rule ID	MSTEST0064
Title	Prefer async assertion methods
Category	Usage
Fix is breaking or non-breaking	Non-breaking
Enabled by default	Yes
Default severity	Info
Introduced in version	4.3.0
Is there a code fix	No

Note

This rule is available starting with MSTest 4.3.

Cause

A test method uses Assert.Throws or Assert.ThrowsExactly to assert an exception is thrown by code that is asynchronous and blocks the asynchronous operation using GetAwaiter().GetResult().

Rule description

When asserting an exception is thrown by asynchronous code, prefer the async assertion methods Assert.ThrowsAsync and Assert.ThrowsExactlyAsync over blocking the asynchronous operation with GetAwaiter().GetResult(). Blocking on async code can cause deadlocks in some synchronization contexts and is harder to read than the equivalent await-based assertion.

[TestClass]
public class TestClass
{
    [TestMethod]
    public async Task Test_ThrowsOnAsyncCall()
    {
        // Violation: blocks the async call inside Assert.Throws.
        Assert.Throws<InvalidOperationException>(() => DoAsync().GetAwaiter().GetResult());
    }

    private static async Task DoAsync()
    {
        await Task.Yield();
        throw new InvalidOperationException();
    }
}

How to fix violations

Use the asynchronous assertion method and await it:

[TestClass]
public class TestClass
{
    [TestMethod]
    public async Task Test_ThrowsOnAsyncCall()
    {
        await Assert.ThrowsAsync<InvalidOperationException>(() => DoAsync());
    }

    private static async Task DoAsync()
    {
        await Task.Yield();
        throw new InvalidOperationException();
    }
}

The same applies to Assert.ThrowsExactly, which has an Assert.ThrowsExactlyAsync counterpart.

When to suppress warnings

You can suppress this warning when the containing test method cannot be made async (for example, a synchronous test overload required by a base class or interface signature) and you need to keep the blocking call.

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 MSTEST0064
// The code that's violating the rule is on this line.
#pragma warning restore MSTEST0064

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

[*.{cs,vb}]
dotnet_diagnostic.MSTEST0064.severity = none

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