VB.NET How give best performance "Select case" or IF... ELSEIF ... ELSE... END IF

vb.net performance if-statement switch-statement cycle
MiBol picture MiBol · Feb 28, 2013 · Viewed 22.7k times · Source

I have a huge DataTable, and I need go by each row and validate an specific value.

Which method give me more performance, an structure of IF ELSE or SELECT CASE? (I'm focused in the method that offer me the best performance)

IF ELSE (METHOD #1)

For Each vRow In vDTtemp.Rows
    If vRow("Item") = "Time" Then
        vRow("Result") = "000"
    ElseIf vRow("Item") = "DateTime" Then
        vRow("Result") = "001"
    ElseIf vRow("Item") = "String" Then
        vRow("Result") = "002"
    Else
        vRow("Result") = "N/A"
    End If
Next

SELECT CASE (METHOD #2)

For Each vRow In vDTtemp.Rows
    Select Case vRow("Item")
        Case "Time"
            vRow("Result") = "000"
        Case "DateTime"
            vRow("Result") = "001"
        Case "String"
            vRow("Result") = "002"
        Case Else
            vRow("Result") = "N/A"
    End Select
Next

Answer

Hans Passant picture Hans Passant · Feb 28, 2013

It makes no difference, both code styles generate the exact same IL. Something you can see by running the ildasm.exe tool on your compiled assembly.

In general, the VB.NET compiler does make an effort to optimize a Select statement. That will work when it uses a simple value type as the selector and trivial Case statements. The generated code will use a dedicated IL instruction, Opcodes.Switch. Which will be compiled to machine code that uses a lookup table. Very fast.

That however doesn't work when you use a string expression as the selector. Making a lookup table for that one would require the equivalent of a dictionary of delegates. That's too impactful, the compiler cannot do anything but convert each case statement to the equivalent of an If statement. You can however optimize it yourself easily by creating this Dictionary in your code, easy to do since the dictionary key and value are just simple strings. You don't have enough cases and the strings are too short to make this pay off majorly, although it is worth a try. It certainly can compact your code.

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