StringBuilder : Can StringBuilder's length & capacity exceed its MaxCapacity

Question

While appending strings to a StringBuilder, can its Capacity and Length go beyond its MaxCapacity?

According to MSDN MaxCapacity is defined as "The maximum number of characters string builder instance can hold". But this behaviour is inconsistent in below two code snippets:

Snippet 1: In the below code ArgumentOutOfRangeException is thrown when the length of the StringBuilder exceeds its MaxCapacity - This is as expected.

        String str = sb.ToString();

        StringBuilder sb1 = new StringBuilder(3, 5);
        sb1.Append("1");      //no error as Length 1 <= max limit 5         
        sb1.Append("12");     //no error as Length 3 <= max limit 5           
        sb1.Append("123");    //ArgumentOutOfRangeException Thrown as Length 6 > max limit 5

Snippet 2: In the below code NO ArgumentOutOfRangeException is thrown when the length of the StringBuilder exceeds its MaxCapacity - This behaviour seems to be incorrect.

        StringBuilder sb = new StringBuilder(3, 5);

        sb.Append("1"); //no error as Length 1 <= max limit 5         
        sb.Append("2"); //no error as Length 2 <= max limit 5         
        sb.Append("3"); //no error as Length 3 <= max limit 5         
        sb.Append("4"); //no error as Length 4 <= max limit 5         
        sb.Append("5"); //no error as Length 5 <= max limit 5         
        sb.Append("6"); //Even though Length 6 > max limit 5 NO EXCEPTION IS THROWN         

        String str = sb.ToString(); //Contains "123456"

Can anyone please explain whats happening in these two cases and why is the difference in the behavior?

Answer 1

StringBuilder Constructor (Int32, Int32)

Notes to Callers
In the .NET Framework 4 and the .NET Framework 4.5, when you instantiate the StringBuilder object by calling the StringBuilder(Int32, Int32) constructor, both the length and the capacity of the StringBuilder instance can grow beyond the value of its MaxCapacity property. This can occur particularly when you call the Append and AppendFormat methods to append small strings.

Additional Resource:

• You can take a look at StringBuilder source code

Conclusion:

This class has written this way for performance reasons and as stated in official documents, its Capacity and Length can grow beyond the its MaxCapacity particularly when appending small strings. Furthermore as stated in documents, some of default values are implementation-specific and so It seems you better don't rely on Capacity and MaxCapacity, and only use this class for performance reasons these conditions:

• When you expect your app to make an unknown number of changes to a string at design time (for example, when you are using a loop to concatenate a random number of strings that contain user input).
• When you expect your app to make a significant number of changes to a string.