String.Concat

  • Thread starter Thread starter John Ford
  • Start date Start date
J

John Ford

For simple string concatenation, is there a difference between...

Dim s As String
s += "add this to string"

....and...

Dim s As String
s = String.Concat(s, "add this to string")

....and...

Dim s As String
s = s + "add this string"

Do they result in the same CLR code? If not, is one faster or
better in some way than the others?
 
John Ford said:
For simple string concatenation, is there a difference between...

Dim s As String
s += "add this to string"

...and...

Dim s As String
s = String.Concat(s, "add this to string")

...and...

Dim s As String
s = s + "add this string"

Do they result in the same CLR code?
Click to expand...

Yes


s = s & ...
and
s &=
also

If not, is one faster or
better in some way than the others?
Click to expand...

No
 
Hello,

John Ford said:
For simple string concatenation, is there a difference between...

Dim s As String
s += "add this to string"

...and...

Dim s As String
s = String.Concat(s, "add this to string")

...and...

Dim s As String
s = s + "add this string"

Do they result in the same CLR code? If not, is one faster or
better in some way than the others?
Click to expand...

Have a look at the code using ildasm.exe.

Notice that it's recommended to use the & operator to concatenate strings in
VB .NET.

HTH,
Herfried K. Wagner
 
John,
Do not use + for string concatenation, use & for string concatenation.

As + is the addition operator while & is the string concatenation operator.

If you use ILDASM.EXE you will see that &= compiles to a call to
String.Concat.

So yes they result in the same IL code, and the speed would be the same.

I normally use the operator as it is a 'cleaner' syntax.

Just remember depending on what you are doing '&=' in a loop for example. It
will be faster to create a System.Text.StringBuilder object and call the
Append method.

For example (VS.NET 2003 syntax)
Dim startTime, endTime As DateTime
Dim time As TimeSpan
Dim s As String
startTime = DateTime.Now
For i As Integer = 0 to 10000
s &= "12345678901234567890"
Next
endTime = DateTime.Now
time = endTime.Subtract(startTime)
Debug.WriteLine(time, "String")

startTime = DateTime.Now
Dim sb As New System.Text.StringBuilder
For i As Integer = 0 to 10000
sb.Append("12345678901234567890")
Next
s = sb.ToString()
endTime = DateTime.Now
time = endTime.Subtract(startTime)
Debug.WriteLine(time, "String")

You will find the first loop takes significantly longer than the second
loop, especially as you increase the number of iterations. The reason for
this is that &= creates a new string for each iteration, while the
StringBuilder maintains a buffer internally that is larger than the
resultant string, this buffer is doubled each time the StringBuilder needs
more room. Resulting in better memory management.

Hope this helps
Jay
 
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