Strings are immutable - meaning the space allocated in memory is fixed. If two strings are concatenated, .net internally creates another string. The string builder is muatble, meaning the space allocated in memory is dynamic.

If two stringbuilders are concatenated, no new object is created. However, if the capacity is reached, then the stringBuilder automatically doubles its capacity (huge).

Example:

Dim szText As System.Text.StringBuilder = New _
System.Text.StringBuilder("First String")

MessageBox.Show("Default Capacity : " & szText.Capacity) ''Returns 16
MessageBox.Show("Max Capacity : " & szText.MaxCapacity)
‘'Returns 2147348647

MessageBox.Show("Default Length : " & szText.Length) ''Returns 12

szText.Append("Second String")
MessageBox.Show("Default Capacity : " & szText.Capacity) ''Returns 32

MessageBox.Show("Max Capacity : " & szText.MaxCapacity)
‘'Returns 2147348647
MessageBox.Show("Default Length : " & szText.Length) ''Returns 25

We can check the capacity
If szText.Capacity < 1024 then
szText.Capacity = 1024
End If

Or

szText.EnsureCapacity (1024)

We can do string manipulation

szText.Insert ( 5, "Middle" ) ''Will insert at the position 5
szText.Replace ( "First", "Begin")