Answer.

String manipulation is one of the most common tasks in Visual Basic. Historically, the String type was initially designed as a convenient mutable object, but was later implemented as immutable to enhance reliability and safety during repetitive data operations. The main issue is that every concatenation operation, when strings are added using + or &, creates a new object — which significantly impacts performance when dealing with large volumes of text, such as when generating reports or logs.

The solution is the use of the StringBuilder class, which allows for efficient "building" of a string while minimizing unnecessary memory allocations and data copying. This class internally maintains a buffer that dynamically expands as needed, and the result can be obtained with a single call to ToString().

Code example:

Dim sb As New System.Text.StringBuilder()
For i As Integer = 1 To 10000
    sb.Append("Line " & i & vbCrLf)
Next
dim result As String = sb.ToString()

Key features:

String — values are immutable, each assignment operation creates a new string in memory.
StringBuilder — efficient for frequent modifications, suitable for concatenating large amounts of data.
Using the + or & operator to concatenate strings is only convenient for small and rare operations.

Trick questions.

Why do "+" and "&" sometimes behave differently when concatenating strings?

The + operator can lead to unexpected type conversions if one of the operands is a number or another type. The & operator strictly concatenates strings, even if the operand is not a string (it gets converted).

Dim a As String = "5"
Dim b As Integer = 10
Dim s1 = a + b    ' This will be 15 due to number addition!
Dim s2 = a & b    ' This will be "510", as string concatenation occurs

Can StringBuilder be slower than String?

Yes, for small strings (for example, 2-3 operations) and rare modifications, StringBuilder can be an overkill — in this case, regular concatenation can be comparable in speed or even faster due to lower memory overhead.

Can a string be modified by index in Visual Basic?

No, strings are immutable — you cannot directly change a character by index. You need to create a new object:

Dim str As String = "test"
str = str.Substring(0, 1) & "A" & str.Substring(2)
' Now str = "tAst"

Common mistakes and anti-patterns

Using the + operator for concatenating a large number of strings in a loop
Expecting that StringBuilder will free memory on its own (use sb.Remove or reassign the object)
Implicit type conversion using +, especially if variables may not be strings

Real-life example

Negative case

In a project, large tables of records were exported line by line using the operator s = s & Line, conducting 10,000+ iterations. Performance significantly dropped with large volumes.

Pros:

Simplicity of the code
Easy to implement without additional libraries

Cons:

High memory usage
Sharp speed drop with large data

Positive case

Replaced concatenation with StringBuilder, collecting all text and converting it to a string only once after the loop.