The StringBuilder Class

The & and &= operators are useful for concatenating a few strings together. However, if you must combine a large number of strings, you may get better performance using the StringBuilder class. This class is optimized for performing long sequences of concatenations to build big strings.

For small pieces of code, the difference between using a String and a StringBuilder is not noticeable. On the one hand, if you need only to concatenate a dozen or so strings once, using a StringBuilder won’t make much difference in run time. On the other hand, if you make huge strings built up in pieces, or if you build simpler strings but many times in a loop, StringBuilder may make your program run faster.

The following code concatenates the string 1234567890 a large number of times, first using a String variable and then using a StringBuilder. In one test that performed the concatenation 10,000 times to build strings 100,000 characters long, using a String took roughly 1.6 seconds. Using a StringBuilder, the program was able to build the string in roughly 0.001 seconds.

  Private Sub btnGo_Click(ByVal sender As System.Object, _  ByVal e As System.EventArgs) Handles btnGo.Click     Const ADD_STRING As String = "1234567890"     Dim num_trials As Long = Long.Parse(txtNumTrials.Text)     Dim start_time As DateTime     Dim stop_time As DateTime     Dim elapsed_time As TimeSpan     Dim txt As String     Dim string_builder As New StringBuilder     lblString.Text = ""     lblStringBuilder.Text = ""     Application.DoEvents()     txt = ""     start_time = Now     For i As Long = 1 To num_trials         txt = txt & ADD_STRING     Next i     stop_time = Now     elapsed_time = stop_time.Subtract(start_time)     lblString.Text = elapsed_time.TotalSeconds.ToString("0.000000")     txt = ""     start_time = Now     For i As Long = 1 To num_trials         string_builder.Append(ADD_STRING)     Next i     txt = string_builder.ToString()     stop_time = Now     elapsed_time = stop_time.Subtract(start_time)     lblStringBuilder.Text = elapsed_time.TotalSeconds.ToString("0.000000") End Sub 

Admittedly, building such enormous strings is not a common programming task. Even when the strings are shorter, you can sometimes see a noticeable difference in performance. The following code concatenates the string 1234567890 to itself 100 times to build a string 1,000 characters long. It builds the string repeatedly for a certain number of trials. In one test building the 1,000 character string 10,000 times, using a String took around 0.95 seconds, whereas using a StringBuilder took about 0.06 seconds.

  Private Sub btnGo_Click(ByVal sender As System.Object, _   ByVal e As System.EventArgs) Handles btnGo.Click     Const ADD_STRING As String = "1234567890"     Dim num_trials As Long = Long.Parse(txtNumTrials.Text)     Dim start_time As DateTime     Dim stop_time As DateTime     Dim elapsed_time As TimeSpan     Dim txt As String     Dim string_builder As New StringBuilder     lblString.Text = ""     lblStringBuilder.Text = ""     Application.DoEvents()     start_time = Now     For i As Long = 1 To num_trials         txt = ""         For j As Long = 1 To 100             txt = txt & ADD_STRING         Next j     Next i     stop_time = Now     elapsed_time = stop_time.Subtract(start_time)     lblString.Text = elapsed_time.TotalSeconds.ToString("0.000000")     txt = ""     start_time = Now     For i As Long = 1 To num_trials         string_builder = New StringBuilder         For j As Long = 1 To 100             string_builder.Append(ADD_STRING)         Next j         txt = string_builder.ToString()     Next i     stop_time = Now     elapsed_time = stop_time.Subtract(start_time)     lblStringBuilder.Text = elapsed_time.TotalSeconds.ToString("0.000000") End Sub  

Strings and string operations are a bit more intuitive than the StringBuilder class, so your code will usually be easier to read if you use String variables when performance isn’t a big issue. If you are building enormous strings, or are building long strings a huge number of times, the performance edge given by the StringBuilder class may be worth slightly more complicated-looking code.