For you to output foo {1, 2, 3} you have to do something like:

string t = "1, 2, 3";
string v = String.Format(" foo {{{0}}}", t);

To output a { you use {{ and to output a } you use }}.

or Now, you can also use c# string interpolation like this (feature available in C# 6.0)

Escaping Brackets: String Interpolation $(""). it is new feature in C# 6.0

var inVal = "1, 2, 3";
var outVal = $" foo {{{inVal}}}";
//Output will be:  foo {1, 2, 3}