Code Inspection: Convert 'as' expression type check and the following null check into pattern matching

tip

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note

This inspection suggests that you use a C# 7.0 feature.

By default, ReSharper automatically detects C# version based on the associated compiler. However, you can specify the target C# version explicitly for a project — right-click the project in the Solution Explorer, choose Edit project item properties from the context menu and use the C# Language Level selector.

To set the C# version for all projects in your solution, specify it in a Directory.Build.props file in your solution directory as described here.

Starting with C# 7.0, we can combine type check, null check, and type cast using pattern matching in the following form: Expression is Type variable — if Expression is not null and assignable to Type, then it will be assigned to variable, otherwise the pattern will return false.

Before C# 7.0, the most widespread pattern to do so was to safe cast the expression using as, and then check for the variable null. ReSharper will detect these patterns in your code and help to convert them into a more elegant pattern matching form. For example:

void Test(object obj)
{
  var str = obj as string;
  if (str != null)
  {
    // do something
  }
}

void Test(object obj)
{
  if (obj is string str)
  {
    // do something
  }
}