Q:

Why is it Valid to Concatenate Null Strings but not to Call "null.ToString()"?

This is valid C# code
var bob = "abc" + null + null + null + "123";  // abc123

This is not valid C# code
var wtf = null.ToString(); // compiler error

Why is the first statement valid?

A:

The reason for first one working:
From MSDN:

In string concatenation operations,the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.

More information on the + binary operator:

The binary + operator performs string concatenation when one or both operands are of type string.
If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object.
If ToString returns null, an empty string is substituted.

The reason of the error in second is:
null (C# Reference) - The null keyword is a literal that represents a null reference, one that does not refer to any object. null is the default value of reference-type variables.

A:

Because the + operator in C# internally translates to String.Concat, which is a static method. And this method happens to treat null like an empty string. If you look at the source of String.Concat in Reflector, you'll see it: 
// while looping through the parameters
strArray[i] = (str == null) ? Empty : str;
// then concatenate that string array

(MSDN mentions it, too: http://msdn.microsoft.com/en-us/library/k9c94ey1.aspx)  
On the other hand, ToString() is an instance method, which you cannot call on null (what type should be used for null?).

A:

The first sample will be translated into:
var bob = String.Concat("abc123", null, null, null, "abs123");

The Concat method checks input and translate null as an empty string
The second sample will be translated into:
var wtf = ((object)null).ToString();

So a null reference exception will be generated here

