C# 6.0 String Interpolation

One of the top ten requests on uservoice is String Interpolation. Today we are going to see how to use this feature and how it is implemented in C# 6.0.

We all use similar expressions:

var str = string.Format("Date: {0}", DateTime.Now);

This is string interpolation in C# before 6.0.

Now, in C# 6.0, we have new string interpolation technique:

var str = $"Date: {DateTime.Now}";

There was nothing new added in to runtime, this is just a syntaxis sugar for "old shcool" interpolation. C# compiler will convert new expression in to "old" one during compilation.

Let's check out more examples:

static void Main(string[] args)
{
    Console.WriteLine($"test {10}");
    //string.Format("test {0}", 10);
    
    var a = 1.2345;           
    Console.WriteLine($"test {a}");
    //string.Format("test {0}", a);

    Console.WriteLine($"test {a} {a} {a}");
    //string.Format("test {0} {1} {2}", a, a, a);

    Console.WriteLine($"test {a + 10}");
    //string.Format("test {0}", a + 10);

    Console.WriteLine($"test {a:00.00}");
    //string.Format("test {0:00.00}", a);

    Console.WriteLine($"test {a + 10:F2}");
    //string.Format("test {0:F2}", a + 10);

    Console.WriteLine($"test {Get10():F2}, {Get0()}");
    //string.Format("test {0:F2}, {1}", Get10(), Get0());

    Console.WriteLine($"test {(args.Length != 0 ? "0" : "1")}");
    //string.Format("test {0}", (args.Length != 0 ? "0" : "1"));

    Console.WriteLine($"test {(args.Length == 0 ? Get0() : "1")}");
    // string.Format("test {0}", (args.Length == 0 ? Get0() : "1"));
}
private static string Get0()
{
    return "0";
}
private static double Get10()
{
    return 876.543;
}

We can check IL code to be shure that this is just a syntax sugar. IL code for first example:

IL_0000:  nop
IL_0001:  ldstr      "Date: {0}"
IL_0006:  call       valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::get_Now()
IL_000b:  box        [mscorlib]System.DateTime
IL_0010:  call       string [mscorlib]System.String::Format(string, object)

As you can see in line IL_0010, there was just a String.Format call.