What is the point of String.Concat(String, String, String, String)

Question

If you have 5 or more strings you'd like to concatenate with String.Concat(), then it uses Concat(String[]).

Why not just use Concat(String[]) all the time and do away with the need for Concat(String, String), Concat(String, String, String) and Concat(String, String, String, String).

What is Microsoft's reason for not just using Concat(String[]) whenever you want to concatenate one or more strings?

Answer 1

According to reference source Concat(String[]) implemented this way:

public static string Concat(params string[] values)
{       
    int totalLength = 0; // variable to calculate total length
    string[] strArray = new string[values.Length]; // second array
    for (int i = 0; i < values.Length; i++) // first loop
    {
        string str = values[i];            
        totalLength += strArray[i].Length;           
    }

    return ConcatArray(strArray, totalLength);
}

private static string ConcatArray(string[] values, int totalLength)
{
    string dest = FastAllocateString(totalLength);
    int destPos = 0; // variable to calculate current position
    for (int i = 0; i < values.Length; i++) // second loop
    {
        FillStringChecked(dest, destPos, values[i]);
        destPos += values[i].Length;
    }
    return dest;
}

Concat(String, String, String) and similar methods are more optimized. They avoid creation of string array for parameters, they avoid loops (each loop has variable, increment and check logic), they don't use variables for calculating total string length and current position in resulting string:

public static string Concat(string str0, string str1, string str2)
{
    int length = (str0.Length + str1.Length) + str2.Length;
    string dest = FastAllocateString(length);
    FillStringChecked(dest, 0, str0);
    FillStringChecked(dest, str0.Length, str1);
    FillStringChecked(dest, str0.Length + str1.Length, str2);
    return dest;
}

NOTE: I skipped parameter validation code (nulls are replaced with empty strings) to show only difference.