String concat with null

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Question 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?

Answer
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.