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

Question

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?

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, Jan 06 '13 at 18:37
@ChrisSinclair Doesn't exist any perversion to use references with lambdas? — Secret, Jan 06 '13 at 19:21
Sorry, I don't understand what you mean, Oleg. Can you rephrase your question please? — Chris Sinclair, Jan 06 '13 at 20:28
@ChrisSinclair I mean this: http://stackoverflow.com/questions/14185982/perversion-does-exist-any-way-to-natively-use-refences-with-lambdas-in-c#comment19663378_14185982 — Secret, Jan 06 '13 at 20:31

Lee · Answer 1 · 2013-01-06T18:09:28.280

2

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.

edited Jan 06 '13 at 18:09

answered Jan 06 '13 at 18:01

Lee

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I think we'll also need to use the `@` prefix on the format string since it appears to be a multi-line string. – Chris Sinclair Jan 06 '13 at 18:06
@lee, @ screening is needed only if you have two (\\) slashes in string or (\") – Secret Jan 06 '13 at 18:16
@OlegOrlov I'm not sure I understand your last comment... You _are_ talking about verbatim strings, right? http://msdn.microsoft.com/en-us/library/aa691090%28VS.71%29.aspx If your string is multi-line (in the souce code file, not because of \r\n escapes), you need the @ prefix. – Chris Sinclair Jan 06 '13 at 18:28
@weston Good point, knew something was itching at the back of my head about mixing those two. – Chris Sinclair Jan 06 '13 at 18:33
Never knew this. However, this breaks his `\r\n` as `\\` is no longer the start of an escape sequence. – weston Jan 06 '13 at 18:35

MBen · Answer 2 · 2013-01-06T18:27:35.477

1

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

edited Jan 06 '13 at 18:27

answered Jan 06 '13 at 18:11

MBen

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Please correct your brackets. They are so many... And i assume, if you use lambda statement you must have `return` in your lambda. – Hamlet Hakobyan Jan 06 '13 at 18:26
@HamletHakobyan thanks, had some misplaced. I corrected the code. – MBen Jan 06 '13 at 18:28
@HamletHakobyan You do not need a `return` in the lambda; it is implied if the lambda has an expression that results in a value and is assigned to a delegate that indicates a return value (such as `Func`) – Chris Sinclair Jan 06 '13 at 18:29
In lambda expression not, but before correction there was lambda statement. – Hamlet Hakobyan Jan 06 '13 at 18:33
@HamletHakobyan Ahh yeah, I see that in the edits; good pick up. – Chris Sinclair Jan 06 '13 at 18:34

score 1 · Answer 3 · answered Jan 06 '13 at 18:31

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);}
        );
    }
}

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

3 Answers3

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