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NCalc is a set of assemblies allowing expressions evaluation. The main class to use is Expression.
This class has a method Evaluate() returning the actual value of its String representation.


  Expression e = new Expression("2 * 3");
  object result = e.Evaluate();
This example above first creates an instance of Expression using a valued constructor. This constructor takes a string as parameter.
Then the method Evaluate() is called to parse the string, and returns the actual value represented by the string.

To create expressions you can combine several operators and values.

Case sensitivity

By default, the evaluation process is case sensitive. This means every parameter and function evaluation will match using case. This behavior can be overriden using a specific evaluation option.

 Expression e = new Expression("aBs(-1)", EvaluateOptions.IgnoreCase)
 Debug.Assert(1M, e.Evaluate());

Handling errors

When the expression has a syntax error, the evaluation will throw an EvaluationException.

     new Expression("(3 + 2").Evaluate();
 catch(EvaluationException e)
     Console.WriteLine("Error catched: " + e.Message);

Though, you can also detect syntax errors before the evaluation by using the HasErrors() method.

 Expression e = new Expression("a + b * (");

Expression caching

When Evaluate() is called on an expression, it is compiled once. If the same expression is reused the compilation is not executed again. Thus you can reuse Expression instances by changing the parameters, and you will gain in performance because only the trasversal of the expression tree will be done.

Moreover, each compiled expression is cached internaly, which means you don't even have to care about reusing an Expression instance, the framework will do it for you. The cache is automatically cleaned like the GC does when an Expression is no more used, or memory is needed (i.e. using WeakReference).

You can disable this behavior at the framework level by setting false to CacheEnabled.

 Expression.CacheEnabled = false;

You can also tell a specific Expression instance not to be taken from the cache.

 Expression e = new Expression("1 + 1", EvaluateOption.NoCache);

Last edited Sep 9, 2009 at 7:50 AM by sebastienros, version 9


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