This topic explains how to customize the error messages produced by the Syntax Parsing Engine.
The following topics are prerequisites to understanding this topic:
This topic contains the following sections:
The Syntax Parsing Engine generates messages when an error is detected. Unless otherwise specified, the error messages will have the following form:
For skipped tokens: “Unexpected token ‘<text from token>’”
For inserted missing nodes:
If the node is a non-terminal symbol: “<NonTerminalSymbol.Name> expected”
If the node is a terminal symbol with a Comparison value of RegularExpression: “<TerminalSymbol.Name> expected”
If the node is any other terminal symbol: “<TerminalSymbol.Value> expected”
For nodes representing non-terminal symbols with their IsError value set to true: “ <NonTerminalSymbol.Name> is incorrect”
In some situations, the default error messages may not be suitable for the purpose of the application. In this case you can customize the messages using one of the methods below:
Override GetErrorAlias method on the LanguageBase derived class
Override the OnError method
When an error occurs, the parser will call the GetErrorAlias method, which allows the language to provide an alias for the specified Symbol. It returns null by default. If you override it and return a non-null value, the error messages will be changed like so:
For skipped tokens: “Unexpected token ‘<alias>’”
For inserted missing nodes: “<alias> expected”
For nodes representing non-terminal symbols with their IsError value set to true: “<alias> is incorrect”
An example of where this might be used is in the Visual Basic language, which has a symbol named “EndGetStatement” to represent the pair of keywords “End Get”. If a user omits an “End Get” at the end of a property get accessor, it might appear odd to the user if the default error message “EndGetStatement expected” were used. Instead, an implementer of the language would probably want to override GetErrorAlias and return ‘End Get’ for this symbol so the error message would appear as “‘End Get’ expected”.
To get more control over the error messages displayed or add or remove error messages, you can alternatively override the OnError method. The ParseErrorContext class, which is an argument to the OnError method, has the following members for allowing you to inspect and customize the error:
For example if you have an “IncompleteMember” error non-terminal defined in C# (see the Error Production Strategy topic for details on this), instead of having an error message that says “IncompleteMember is incorrect”, you might want to replace it entirely by saying “Incomplete member definition in class or struct”.
The following topics provide additional information related to this topic.