What does {{{0}}} on string.Format do?

Question

In the namespace MS.Internal, there is a class named NamedObject.

It has a weird block of code:

public override string ToString() {   if (_name[0] != '{')   {     // lazily add {} around the name, to avoid allocating a string      // until it's actually needed     _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);   }    return _name; } 

I'm curious about this comment specifically:

    // lazily add {} around the name, to avoid allocating a string      // until it's actually needed     _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name); 

How is that 'lazy'? What does it do to be lazy?

---

Full class from the reference source:

//----------------------------------------------------------------------------  // // <copyright file="NamedObject.cs" company="Microsoft"> //    Copyright (C) Microsoft Corporation.  All rights reserved. // </copyright>  // // Description: Placeholder object, with a name that appears in the debugger  //  //---------------------------------------------------------------------------  using System; using System.Globalization; using MS.Internal.WindowsBase;  namespace MS.Internal {   /// <summary>    /// An instance of this class can be used wherever you might otherwise use   /// "new Object()".  The name will show up in the debugger, instead of    /// merely "{object}"   /// </summary>   [FriendAccessAllowed]   // Built into Base, also used by Framework.   internal class NamedObject   {     public NamedObject(string name)     {       if (String.IsNullOrEmpty(name))         throw new ArgumentNullException(name);        _name = name;     }      public override string ToString()     {       if (_name[0] != '{')       {         // lazily add {} around the name, to avoid allocating a string          // until it's actually needed         _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", _name);       }        return _name;     }      string _name;   } }  // File provided for Reference Use Only by Microsoft Corporation (c) 2007. // Copyright (c) Microsoft Corporation. All rights reserved. 

Answer 1

You escape a curly brace with a curly brace, i.e. {{ produces {, and }} produces }.

The {0} in the middle is interpreted as usual - i.e. a reference to parameter at index zero.

{{ {0} }} ^^ ^^^ ^^ |   |  | |   |  +--- Closing curly brace |   +------ Parameter reference +---------- Opening curly brace 

The end result is the value of parameter zero enclosed in curly braces:

var res = string.Format("{{{0}}}", "hello"); // produces {hello} 

How is that 'lazy'?

They call it lazy with respect to this alternative "eager" implementation:

internal class NamedObject {     public NamedObject(string name) {         if (String.IsNullOrEmpty(name))             throw new ArgumentNullException(name);         if (name[0] != '{') {             // eagerly add {} around the name             _name = String.Format(CultureInfo.InvariantCulture, "{{{0}}}", name);         } else {             _name = name;         }     }     public override string ToString() {         return _name;     }     string _name; } 

This implementation adds curly braces right away, even though it has no idea that the name enclosed in curly braces is going to be needed.