Tukukkk

I want to create an error message string. It should contain multiple hints to fix the errors.

First I created something like this

string errorMessage = string.Empty;



if (1 == 1)

    errorMessage += "- hint 1n";



if (2 == 2)

    errorMessage += "- hint 2n";



if (3 == 3)

    errorMessage += "- hint 3";



// do something with errorMessage

And I thought about cleaning it up. I created an extension method

public static void AppendIf(this string s, bool condition, string txtToAppend)

{

    if (condition)

        s += txtToAppend;

}

And call it within my class

string errorMessage = string.Empty;



errorMessage.AppendIf(1 == 1, "- hint 1n");

errorMessage.AppendIf(2 == 2, "- hint 2n");

errorMessage.AppendIf(3 == 3, "- hint 3");



// do something with errorMessage

But errorMessage stays empty. I thought this acts like the ref keyword so what is wrong with my extension method?

You could not use this ref modifier, please check compiler feature request.

However you can get the same result by using the StringBuilder type:

public static void AppendIf(this StringBuilder s, bool condition, string txtToAppend)

{

    if (condition)

        s.Append(txtToAppend);

}

So, your code will be:

string errorMessage = new StringBuilder();



errorMessage.AppendIf(1 == 1, "- hint 1n");

errorMessage.AppendIf(2 == 2, "- hint 2n");

errorMessage.AppendIf(3 == 3, "- hint 3");

NB: please avoid executing code like str += anotherStr; inside the loop, because this method has O(N^2) complexity, where N is count of chars. Please check details in this question.

string is immutable, which means that it creates a new string every time you append to it, so it is impossible.

However, you can use StringBuilder to achieve it:

public static class StringBuilderExtensions

{

    public static StringBuilder AppendLineIf(this StringBuilder builder, bool condition, string line)

    {

        // validate arguments

        if (condition) 

            builder.AppendLine(line);

        return builder;            

    }



    public static StringBuilder AppendIf(this StringBuilder builder, bool condition, string line)

    {

        // validate arguments

        if (condition) 

            builder.Append(line);

        return builder;            

    }

}



StringBuilder builder = new StringBuilder();

builder.AppendLineIf(1 == 1, "- hint 1");

builder.AppendLineIf(2 == 2, "- hint 2");

builder.AppendLineIf(3 == 3, "- hint 3");



string result = builder.ToString();

// do something with result

You can also chain these calls, if it looks better for you:

string result = new StringBuilder()

    .AppendLineIf(1 == 1, "- hint 1")

    .AppendLineIf(2 == 2, "- hint 2")

    .AppendLineIf(3 == 3, "- hint 3")

    .ToString();