The Fix Format String quick fix command is available for String.Format and StringBuilder AppendFormat methods.
It commonly appears when the format items of the composite format string are not zero-based indexed placeholders or there are missing indexes.
Supported: C#, VB.NET
The behavior can be controlled in Problems option's page -
JustCode / Options / Code Analysis / Problems / C#.
When "Invalid format string defined" problem is checked as warning or error in the options dialog,
then Telerik® JustCode™ suggests you to use Fix Format String.
To use Fix Format String :
Position the caret inside the undelined string.
Select Fix Format String from the VisualAid's Fixes menu.
You can press Alt+Enter in order to expand the Fixes menu.
The result is (when applied for each expression):