Checking is StringBuilder() AppendLine() is an empty string - vb.net

Question

I want to conditionally append a string, derived from a function, to a string builder. The required condition is that the function is not returning a blank string ("").
The purpose of conditionally appending the string is to avoid AppendLine() appending a line when the string (returned from the function) being appended is empty.
My current code (wrapping the function in a condition calling the very same function):
Dim builder As New System.Text.StringBuilder()
builder.Append("Some text...").AppendLine()
If Not IsNothing(someFunction(someParameterAA, someParameterAB)) Then
builder.Append(someFunction(someParameterAA, someParameterAB)).AppendLine()
End If
If Not IsNothing(someFunction(someParameterBA, someParameterBB)) Then
builder.Append(someFunction(someParameterBA, someParameterBB)).AppendLine()
End If
builder.AppendLine().Append("...some text.")
Dim s As String = builder.ToString
MessageBox.Show(s)
It is my hope that a more efficient alternative exists (efficient in terms of the amount of code to be written). Ultimately, I'd like to avoid calling the same function twice however I cannot independently add the builder.Append line of code to my function. Is it instead possible to target builder.Append?
Example of the potential logic:
If `builder.Append()` inside the following brackets is not an empty string then:
(
builder.Append(someFunction(someParameterAA, someParameterAB)).AppendLine()
)
If anyone has a solution on the above, bear in mind the prequisite of concision (=< 2) lines of code additional to the builder.Append() line.
I'm open to any other suggestions.

Answer 1

Create another method to do the appending like so:
CheckBeforeAppend(someFunction(someParameterAA, someParameterAB), builder)
CheckBeforeAppend(someFunction(someParameterBA, someParameterBB), builder)
....
Public Sub CheckBeforeAppend(s As String, sb As StringBuilder)
If Not String.IsNullOrEmpty(s)
sb.Append(s).AppendLine()
End If
End Sub
A simple refactor such as this shortens your original code and means you don't need to duplicate the null or empty check on the return value of your function.
And for the extension method (place this code in a Module):
<Extension()>
Public Sub CheckBeforeAppend(s As String, sb As StringBuilder)
If Not String.IsNullOrEmpty(s)
sb.Append(s).AppendLine()
End If
End Sub
usage:
someFunction(someParameterAA, someParameterAB).CheckBeforeAppend(sb)
or for an extension on StringBuilder:
<Extension()>
Public Sub CheckBeforeAppend(sb As StringBuilder, s As String)
If Not String.IsNullOrEmpty(s)
sb.Append(s).AppendLine()
End If
End Sub
usage:
builder.CheckBeforeAppend(someFunction(someParameterAA, someParameterAB))

Answer 2

You can avoid calling the function twice by storing the result of the function in a variable.
Dim myString As String = someFunction(someParameterAA, someParameterAB)
If myString <> "" Then
builder.Append(myString).AppendLine()
End If
myString = someFunction(someParameterBA, someParameterBB)
If myString <> "" Then
builder.Append(myString).AppendLine()
End If
This way you just call the function once and check your conditions with the variable. Also the code looks a lot smaller and more understandable.