What is this?

A Roslyn analyzer that allows you to enforce a no-capture policy for passing lambdas to your methods by decorating them with a special attribute.

How to use it?

1. Install the HellBrick.NoCapture package from NuGet

2. Decorate the methods or the parameters that you don't want the callers to pass capturing lambdas to with the [NoCapture] attribute:

[NoCapture]
public static IEnumerable<TOut> Select<TIn, TOut, TArg>( this IEnumerable<TIn> sequence, TArg argument, Func<TIn, TArg, TOut> selector )
{
	foreach ( TIn item in sequence )
		yield return selector( item, argument );
}

public static IEnumerable<TOut> Select<TIn, TOut>( this IEnumerable<TIn> sequence, [NoCapture] Func<TIn, TOut> selector )
		=> sequence.Select( selector, ( item, innerSelector ) => innerSelector( item ) );

// ...

public class Caller
{
	private readonly int[] _numbers = { 64, 128, 256 };
	private readonly int _field = 42;

	public void CallSite( int argument )
	{
		// Error: Select( selector ) requires a non-capturing lambda. Captured variables: this.
		_numbers.Select( x => x + _field );

		// Error: Select( selector ) requires a non-capturing lambda. Captured variables: this, argument.
		_numbers.Select( "", ( x, _ ) => x + _field + argument );
	}
}

Limitations

At the moment, only the lambdas defined inline in the argument list are checked for the violations of the [NoCapture] attribute. For instance, the following call site, if added to the previous example, will trick the analyzer into a false negative:

public void AnotherCallSite()
{
	// No error yet.
	Func<int, int> lambda = x => x + _field;
	_numbers.Select( lambda );
}