Lambda expression C# failed to convert to the object type and fail with ref

问题

Have such function:

   private static void EncodeString(ref string str)
    {
        using (RLE inst_rle = new RLE())
        {
            string str_encoded = inst_rle.Encode(ref str);
            Console.WriteLine(

              "\r\nBase string ({0} chars): {1}\r\nAfter RLE-encoding ({2} chars): {3}\r\nCompression percentage: %{4}",
              str.Length, str, str_encoded.Length, str_encoded,
              () => { (100 * (str.Length - str.encoded.Length) / str.Length); }

                             );
        }
    }

As I remember it's a style of lambdas in C#: () => { < action > ; }

But getting such errors:

Cannot convert lambda expression to type 'object' because it
Only assignment, call, increment, decrement, and new object expressions can be used as a statement
Cannot use ref or out parameter 'str' inside an anonymous method, lambda expression, or query expression
Cannot use ref or out parameter 'str' inside an anonymous method, lambda expression, or query expression

How to use Lambda in C# EXACLTY in my app (console app) without explicity using

Delegate / Func< T >, like in () => { } way?

回答1:

I'm not really sure why you want to use a lambda here, it looks like you want:

Console.WriteLine(@"\r\nBase string ({0} chars): {1}\r\nAfter RLE-encoding
            (
             {2} chars): {3}\r\nCompression percentage: {4}",
             str.Length, str, str_encoded.Length, str_encoded,
             (100 / str.Length) * (str.Length / str_encoded.Length)
            );

As the comment points out, you need to prefix the format string with an @ since it spans multiple lines.

回答2:

I agree with Lee, but when you really want to create a Lamba like this, and get its output you need to cast explicitly something like:

(Func<int>)(() => (100 / str.Length) * (str.Length / str_encoded.Length)))();

I do this when I am playing with threads, not in production code though

回答3:

String constants can be defined over multiple code lines with a @ prefix, but then your \r\n will not work. So instead you can add string framgents together with + to achieve the same effect:

private static void EncodeString(ref string str)
{
    using (RLE inst_rle = new RLE())
    {
        string str_encoded = inst_rle.Encode(ref str);
        Console.WriteLine("\r\nBase string ({0} chars): {1}\r\nAfter RLE-encoding" +
        "(" +
         "{2} chars): {3}\r\nCompression percentage: {4}",
         str.Length, str, str_encoded.Length, str_encoded,
         () => { (100 / str.Length) * (str.Length / str_encoded.Length);}
        );
    }
}

来源：https://stackoverflow.com/questions/14185007/lambda-expression-c-sharp-failed-to-convert-to-the-object-type-and-fail-with-ref

标签

linq

lambda

expression