Question

This loop takes forever to run as the amount of items in the loop approach anything close to and over 1,000, close to like 10 minutes. This needs to run fast for amounts all the way up to like 30-40 thousand.

'Add all Loan Record Lines
Dim loans As List(Of String) = lar.CreateLoanLines()
Dim last As Integer = loans.Count - 1
For i = 0 To last
    If i = last Then
        s.Append(loans(i))
    Else
        s.AppendLine(loans(i))
    End If
Next

s is a StringBuilder. The first line there

Dim loans As List(Of String) = lar.CreateLoanLines()

Runs in only a few seconds even with thousands of records. It’s the actual loop that’s taking a while.

How can this be optimized???

Answer 1

Set the initial capacity of your StringBuilder to a large value. (Ideally, large enough to contain the entire final string.) Like so:

s = new StringBuilder(loans.Count * averageExpectedStringSize)

If you don’t specify a capacity, the builder will likely end up doing a large amount of internal reallocations, and this will kill performance.