The most performant way of appending two string builders in .NET Framework 4, use this;
frontStringBuilder.Append(backStringBuilder);
For .NET Framework 2.0/3.5, use this;
frontStringBuilder.Append(backStringBuilder.ToString(0, backStringBuilder.Length));
This wouldn’t hurt performance in .NET FW 4.0.
Resources
https://stackoverflow.com/questions/6454392/how-to-append-two-stringbuilders
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