Does String.Replace() create a new string if there's nothing to replace?

Question

For example:

public string ReplaceXYZ(string text)
{
    string replacedText = text;

    replacedText = replacedText.Replace("X", String.Empty);
    replacedText = replacedText.Replace("Y", String.Empty);
    replacedText = replacedText.Replace("Z", String.Empty);

    return replacedText;
}

If I were to call "ReplaceXYZ" even for strings that do not contain "X", "Y", or "Z", would 3 new strings be created each time?

I spotted code similar to this in one of our projects. It's called repeatedly as it loops through a large collection of strings.

Answer 1

It does not return a new instance if there is nothing to replace:

string text1 = "hello world", text2 = text1.Replace("foo", "bar");
bool referenceEqual = object.ReferenceEquals(text1, text2);

After that code executes, referenceEqual is set to true.

Even better, this behavior is documented:

If oldValue is not found in the current instance, the method returns the current instance unchanged.

Otherwise, this would be implementation-dependent and could change in the future.

Note that there is a similar, documented optimization for calling Substring(0) on a string value:

If startIndex is equal to zero, the method returns the original string unchanged