Answer.

Strings (String) in Visual Basic .NET are immutable. Any operation, such as concatenation, results in the creation of a new string rather than modifying the existing one. When there are many concatenations/changes to strings, it leads to noticeable performance drops and increased memory usage.

For working with large amounts of text, it is recommended to use special classes for efficient string manipulation. In Visual Basic .NET, this is System.Text.StringBuilder, which allows modifying the string buffer without constantly allocating new memory.

Example with StringBuilder:

Imports System.Text
Dim builder As New StringBuilder()
For i As Integer = 1 To 100000
    builder.Append("String " & i & vbCrLf)
Next
Dim result As String = builder.ToString()

Trick question.

Question: "Can we just concatenate strings using the & operator when working with large text files and not worry about performance?"

Answer: No. If you concatenate strings in a loop using &, a new string object is created for each iteration and all its characters are additionally copied. This is inefficient for large amounts of data. It is better to use StringBuilder.

Example:

' This code is very slow for large N:
Dim s As String = ""
For i = 1 To 1000000
    s = s & "a"
Next
' And this one is much faster:
Dim sb As New StringBuilder()
For i = 1 To 1000000
    sb.Append("a")
Next
Dim s = sb.ToString()

Examples of real errors due to ignorance of the nuances of the topic.

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Story:

In a web application, the project was collecting email newsletters by simply concatenating strings in a loop using &. At volumes greater than 10,000 messages, the system began to "hang" and crashed due to memory overflow. After switching to StringBuilder, these issues disappeared.

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Story:

In the error logging procedure, logs were collected by repeatedly adding strings through regular addition. This sharply increased the log writing time and slowed down the entire parser's operation. After auditing and refactoring with StringBuilder, the speed increased fivefold.

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Story:

When generating reports in popular XLS files, the analyst chose a simple approach — concatenating all values into one string using &. The result: when generating a report of 10,000 rows, the application became unresponsive. Replacing it with StringBuilder solved the performance and memory leak issues.