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

0
votes

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?

3
c#linqlambdaexpression
FYI, I don't think any of the answers have picked up that you can't use your str parameter in any of the lambdas. As str is marked with ref, you can't use it in the lambda. You would have to store it as a local temporary variable first before using in the lambda. As Lee pointed out though, I see no reason to be using a lambda at all in this case anyway. - Chris Sinclair
@ChrisSinclair Doesn't exist any perversion to use references with lambdas? - Secret
Sorry, I don't understand what you mean, Oleg. Can you rephrase your question please? - Chris Sinclair
@ChrisSinclair I mean this: stackoverflow.com/questions/14185982/… - Secret

3 Answers

2
votes

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.

1
votes

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

1
votes

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);}
        );
    }
}