About dynamic expressions and dynamic evaluation

In a dynamic expression, the actual expression, not just its variable values, is determined at execution time. In other words, in a dynamic expression the structure of the expression, such as the names of the variables, not just the values of the variables, gets built at runtime.

You create dynamic expressions using string expressions, which are expressions contained in strings, (that is, surrounded with quotation marks). Dynamic evaluation is the process of evaluating a string expression. The Evaluate and IIf functions, and only these functions, perform dynamic evaluation.

When ColdFusion performs dynamic evaluation it does the following:

  1. Takes a string expression and treats it as a standard expression, as if the expression was not a string.

  2. Parses the expression to determine the elements of the expression and validate the expression syntax.

  3. Evaluates the expression, looking up any variables and replacing them with their values, calling any functions, and performing any required operations.

This process enables ColdFusion to interpret dynamic expressions with variable parts. However, it incurs a substantial processing overhead.

Dynamic expressions were important in early versions of ColdFusion, before it supported arrays and structures, and they still can be useful in limited circumstances. However, the ability to use structures and the ability to use associative array notation to access structure elements provide more efficient and easier methods for dynamically managing data. For information on using arrays and structures, see Using Arrays and Structures.