[c#] Adding line break in C# Code behind page

Strings are immutable, so using

public string GenerateString()
{
    return
        "abc" +
        "def";
}

will slow you performance - each of those values is a string literal which must be concatenated at runtime - bad news if you reuse the method/property/whatever alot...

Store your string literals in resources is a good idea...

public string GenerateString()
{
    return Resources.MyString;
}

That way it is localisable and the code is tidy (although performance is pretty terrible).