Question

When using a string builder, I get expected results from the append and append line functions when using an enum is the input, but when the enum is boxed the append line and append function give differing results.

Can anyone tell me what may be behind this?

Code output:

Append Enum: 1
Append Enum To String: One
Append Line Enum: 1
Append Line Enum To String: One
Append Object: One
Append Object To String: One
Append Line Object: 1
Append Line Object To String: One

Code:

Public Enum eTest
    One = 1
    Two = 2
End Enum

Sub Main()
    Dim sb As New System.Text.StringBuilder()
    Dim x = eTest.One
    sb.Append("Append Enum: ").Append(x).AppendLine()
    sb.Append("Append Enum To String: ").Append(x.ToString()).AppendLine()
    sb.Append("Append Line Enum: ").AppendLine(x)
    sb.Append("Append Line Enum To String: ").AppendLine(x.ToString())

    Dim o As Object = x
    sb.Append("Append Object: ").Append(o).AppendLine()
    sb.Append("Append Object To String: ").Append(o.ToString()).AppendLine()
    sb.Append("Append Line Object: ").AppendLine(o)
    sb.Append("Append Line Object To String: ").AppendLine(o.ToString())

    Console.WriteLine(sb.ToString())


    '===============================
    Console.ReadKey()
End Sub

Answer 1

This line

sb.Append("Append Object: ").Append(o).AppendLine()

causes o.ToString() to be appended to the instance of StringBuilder and that is the difference. Of course, that call just passes through to the boxed instance of your enum and this is why you see One for this invocation.

This is because you are invoking the overload StringBuilder.Append(object) which will just invoke o.ToString() on the passed in instance of object. This is explicitly in the documentation:

The Append method calls the Object.ToString method to get the string representation of value. If value is null, no changes are made to the StringBuilder object.

On the other hand, when you call

sb.Append("Append Enum: ").Append(x).AppendLine()

where x is an instance of an enum, x will be implicitly converted to an instance of int which will call the overload StringBuilder.Append(int).

Note that this is specific to VB.NET. C# will not implicitly convert x to an int when choosing which overload of StringBuilder.Append to invoke; it will invoke StringBuilder.Append(object).