NUnit2057

Remove unnecessary lambda expression

Description

Only the argument value is needed by the Assert.

Motivation

The Assert method will call a delegate if passed in, but that is an unnecessary overhead.

In most cases, only the return value is needed by the assertion.

There are two exceptions:

1. When catching exceptions, like in:

   csharpCopy

   Assert.That(() => instance.Method1(null), Throws.ArgumentNullException);
   

2. When a method might need to be called multiple times, like in:

   csharpCopy

   Assert.That(() => task.IsCompleted, Is.True.After(5).Seconds.PollEvery(100).MilliSeconds);
   

Sometimes developers unnecessarily create a lambda expression for the actual parameter.

Assert.That(() => instance.Method1(Parameters), Is.EqualTo(expected));

Here, the call is performed inside the Assert and then the result is tested. This is not necessary. The same test can be written as:

Assert.That(instance.Method1(Parameters), Is.EqualTo(expected));

It is clearer both in looks and in what is being tested here.

Sometimes the lambda is created implicitly like in:

Assert.That(instance.Method2, Is.EqualTo(expected));

The fixed code should look like:

Assert.That(instance.Method2(), Is.EqualTo(expected));

This clearly indicates that the Assert is on the return value of the method.

How to fix violations

Remove the anonymous lambda creation () =>. There is a code-fix that can do this for you.

Configure severity

Via ruleset file

Configure the severity per project, for more info see MSDN.

Via .editorconfig file

# NUnit2057: Remove unnecessary lambda expression
dotnet_diagnostic.NUnit2057.severity = chosenSeverity

where chosenSeverity can be one of none, silent, suggestion, warning, or error.

Via #pragma directive

#pragma warning disable NUnit2057 // Remove unnecessary lambda expression
Code violating the rule here
#pragma warning restore NUnit2057 // Remove unnecessary lambda expression

Or put this at the top of the file to disable all instances.

#pragma warning disable NUnit2057 // Remove unnecessary lambda expression

Via attribute [SuppressMessage]

[System.Diagnostics.CodeAnalysis.SuppressMessage("Assertion",
    "NUnit2057:Remove unnecessary lambda expression",
    Justification = "Reason...")]