Should I use string.Contains() before string.Replace()?

Question

Is it unnecessary to have this if statement before doing a string replace?

 if (myString.Contains(oldValue))
 {
      myString = myString.Replace(oldValue, newValue);
 }

Answer 1

All the details are in the documentation for String.Replace:

Return Value:
A string that is equivalent to the current string except that all instances of oldValue are replaced with newValue. If oldValue is not found in the current instance, the method returns the current instance unchanged.

The if statement is not required.

An if statement is not even a performance optimization, since String.Replace returns the same object instance, if oldValue is not found. I have verified this using the following code:

namespace StringReplaceTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "Test";
            string s2 = s.Replace("Foo", "Bar");
            string s3 = s.Replace("es", "tt");
        }
    }
}

Using the handy Make Object ID feature (right-click on a symbol in the Locals, Auto, or Watch window; see Common Expression Evaluator Features for more details) produced the following output:

s  | "Test" {$1}
s2 | "Test" {$1}
s3 | "Tttt" {$2}