In C#, should I use string.Empty or String.Empty or "" to intitialize a string?

There really is no difference from a performance and code generated standpoint. In performance testing, they went back and forth between which one was faster vs the other, and only by milliseconds.

In looking at the behind the scenes code, you really don't see any difference either. The only difference is in the IL, which string.Empty use the opcode ldsfld and "" uses the opcode ldstr, but that is only because string.Empty is static, and both instructions do the same thing. If you look at the assembly that is produced, it is exactly the same.

C# Code

private void Test1()
{
    string test1 = string.Empty;    
    string test11 = test1;
}

private void Test2()
{
    string test2 = "";    
    string test22 = test2;
}

IL Code

.method private hidebysig instance void 
          Test1() cil managed
{
  // Code size       10 (0xa)
  .maxstack  1
  .locals init ([0] string test1,
                [1] string test11)
  IL_0000:  nop
  IL_0001:  ldsfld     string [mscorlib]System.String::Empty
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  stloc.1
  IL_0009:  ret
} // end of method Form1::Test1

.method private hidebysig instance void 
        Test2() cil managed
{
  // Code size       10 (0xa)
  .maxstack  1
  .locals init ([0] string test2,
                [1] string test22)
  IL_0000:  nop
  IL_0001:  ldstr      ""
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  stloc.1
  IL_0009:  ret
} // end of method Form1::Test2

Assembly code

        string test1 = string.Empty;
0000003a  mov         eax,dword ptr ds:[022A102Ch] 
0000003f  mov         dword ptr [ebp-40h],eax 

        string test11 = test1;
00000042  mov         eax,dword ptr [ebp-40h] 
00000045  mov         dword ptr [ebp-44h],eax 

        string test2 = "";
0000003a  mov         eax,dword ptr ds:[022A202Ch] 
00000040  mov         dword ptr [ebp-40h],eax 

        string test22 = test2;
00000043  mov         eax,dword ptr [ebp-40h] 
00000046  mov         dword ptr [ebp-44h],eax 

One difference is that if you use a switch-case syntax, you can't write case string.Empty: because it's not a constant. You get a Compilation error : A constant value is expected

Look at this link for more info: string-empty-versus-empty-quotes

The best code is no code at all:

The fundamental nature of coding is that our task, as programmers, is to recognize that every decision we make is a trade-off. […] Start with brevity. Increase the other dimensions as required by testing.

Consequently, less code is better code: Prefer "" to string.Empty or String.Empty. Those two are six times longer with no added benefit — certainly no added clarity, as they express the exact same information.

Use whatever you and your team find the most readable.

Other answers have suggested that a new string is created every time you use "". This is not true - due to string interning, it will be created either once per assembly or once per AppDomain (or possibly once for the whole process - not sure on that front). This difference is negligible - massively, massively insignificant.

Which you find more readable is a different matter, however. It's subjective and will vary from person to person - so I suggest you find out what most people on your team like, and all go with that for consistency. Personally I find "" easier to read.

The argument that "" and " " are easily mistaken for each other doesn't really wash with me. Unless you're using a proportional font (and I haven't worked with any developers who do) it's pretty easy to tell the difference.