How does C# string interpolation without an expression compile?

Question

How does the compiler handle interpolated strings without an expressions?

string output = $"Hello World";

Will it still try to format the string? How does the compiled code differ from that of one with an expression?

Answer 1

For this C# code:

string output = $"Hello World";

int integer = 5;

string output2 = $"Hello World {integer}";

Console.WriteLine(output);

Console.WriteLine(output2);

I get this when I compile and then decompile (via ILSpy):

string value = "Hello World";
int num = 5;
string value2 = string.Format("Hello World {0}", num);
Console.WriteLine(value);
Console.WriteLine(value2);

So it seems that the compiler is smart enough not to use string.Format for the first case.

For completeness, here is the IL code:

IL_0000: nop
IL_0001: ldstr "Hello World"
IL_0006: stloc.0
IL_0007: ldc.i4.5
IL_0008: stloc.1
IL_0009: ldstr "Hello World {0}"
IL_000e: ldloc.1
IL_000f: box [mscorlib]System.Int32
IL_0014: call string [mscorlib]System.String::Format(string, object)
IL_0019: stloc.2
IL_001a: ldloc.0
IL_001b: call void [mscorlib]System.Console::WriteLine(string)
IL_0020: nop
IL_0021: ldloc.2
IL_0022: call void [mscorlib]System.Console::WriteLine(string)
IL_0027: nop
IL_0028: ret

It is clear here too that string.Format is only called for the second case.