Question

Reflector tells me that SortedList uses a ThrowHelper class to throw exceptions instead of throwing them directly, for example:

public TValue this[TKey key] {     get     {         int index = this.IndexOfKey(key);         if (index >= 0)             return this.values[index];         ThrowHelper.ThrowKeyNotFoundException();         return default(TValue);     } 

where ThrowKeyNotFoundException does nothing more than just:

throw new KeyNotFoundException(); 

Note how this requires a duff statement ‘return default(TValue)’ which is unreachable. I must conclude that this is a pattern with benefits large enough to justify this.

What are these benefits?

Answer 1

According to ThrowHelper.cs source code the main purpose is to reduce the JITted code size. Below is a direct copy paste from the link:

// This file defines an internal class used to throw exceptions in BCL code. // The main purpose is to reduce code size.  //  // The old way to throw an exception generates quite a lot IL code and assembly code. // Following is an example: //     C# source //          throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key")); //     IL code: //          IL_0003:  ldstr      "key" //          IL_0008:  ldstr      "ArgumentNull_Key" //          IL_000d:  call       string System.Environment::GetResourceString(string) //          IL_0012:  newobj     instance void System.ArgumentNullException::.ctor(string,string) //          IL_0017:  throw //    which is 21bytes in IL. //  // So we want to get rid of the ldstr and call to Environment.GetResource in IL. // In order to do that, I created two enums: ExceptionResource, ExceptionArgument to represent the // argument name and resource name in a small integer. The source code will be changed to  //    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key, ExceptionResource.ArgumentNull_Key); // // The IL code will be 7 bytes. //    IL_0008:  ldc.i4.4 //    IL_0009:  ldc.i4.4 //    IL_000a:  call       void System.ThrowHelper::ThrowArgumentNullException(valuetype System.ExceptionArgument) //    IL_000f:  ldarg.0 // // This will also reduce the Jitted code size a lot.