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gistfile1.txt

@FunctionalInterface
public interface Assertion<TArg1, TArg2, TArg3> {

    void Execute(TArg1 arg1, TArg2 arg2, TArg3 arg3);
}

public static Assertion<RestContextBase, RestRequest, RestResponse> CreatedWithJsonBody() {

    return (RestContextBase context, RestRequest y, RestResponse z)  -> {

        context.AssertStatusCode(y, z, 201); // 201 CREATED
        context.AssertContentType(y, z, "application/json");
        context.AssertContentLength(y, z, ContentLength.NonZero);
    };
}

You have to specify the type params for Assertion.  Just like in C#.