C# Eval Expression What is the Eval Context?

The EvalContext is the main class of the C# Eval Expression library. No matter how you call the Execute or Compile methods, an EvalContext is used.

There are 2 ways to use an EvalContext:

  • Instance Context: When you create a new instance of the EvalContext class
  • Global Context: When you use a method that uses under the hood the EvalManager.DefaultContext.

Instance Context

The instance context is the common usage you know from a class. You create a new instance of the EvalContext and directly use properties and methods from it:

// CREATE a new instance of EvalContext
var context = new EvalContext();

// USE the `Execute` method from the context created
var list1 = context.Execute<List<int>>("new List<int>() { 1, 2, 3 };");

FiddleHelper.WriteTable(list1);

Try it online

Global Context

The global context is an instance stored in a static variable in the EvalManager (EvalManager.DefaultContext).

We call it the global context because:

  • You can access this context anywhere in your code without creating an instance
  • Multiple methods of our library not related to a context use this global context under the hood, such as:
    • Eval.Execute(code)
    • "code".Execute()
    • linq.Execute(code)
var list2 = Eval.Execute<List<int>>("new List<int>() { 1, 2, 3 };");

var list3 = "new List<int>() { 1, 2, 3 };".Execute<List<int>>();

Try it online

How to customize a Context?

You can customize the instance and global context with

  1. Options
  2. Register / Unregister Methods

1. Options

Customizing your context with options allows you to change how an expression is parsed and executed. Here are a few examples:

  • IsCaseSensitive: Allow you to set if properties and methods names in the expression should respect the casing
  • MaxLoopIteration: Allow you to set a maximum of iterations to avoid an infinite loop
  • SafeMode: Allow you to restrict the usage of our library to only what you want the user to do

See the Options documentation for more information and examples.

2. Register / Unregister Methods

By default, a context doesn't know about types, variables, and constants in your project and referenced projects. So if you have a Customer type and when to use it in your expression, you must first register it.

Here is a short list of what you can register in the context:

  • Register a type
  • Register a static method
  • Register a constant
  • Register a global variable
  • Register a keyword

See the Register / Unregister Methods documentation for more information and examples.

When to use an Instance Context vs. the Global Context?

The usual answer we provide is:

  • Instance Context: If you need to customize the context for this specific evaluation, you should use an instance context
  • Global Context: If you don't need to customize the context for this specific evaluation, you can use the global context

All methods, options, and registers methods are thread-safe within the C# Eval Expression library, but being thread-safe doesn't mean you can necessarily use it in any way in a concurrency scenario.

For example, is a ConcurrencyDictionary thread-safe? Sure, but if Thread A adds a key and Thread B tries to remove the key if it exists, will the key still exist for Thread A? The same problem happens when you change options, register or unregister type.

Configuring the global context should only happen once, for example, when the application starts and never after. This is why we say:

  • You should: use an instance context if you need to customize the context. Otherwise, it can lead to some side impact
  • You can: use the global context if you don't need to customize the context. There is no problem with using an instance context either in this case

The #1 problem by far we see by people using this library is that they keep modifying the global context without thinking that it could impact another thread that will eventually compile an expression.

Conclusion

The EvalContext is the class controlling almost all the logic in the C# Eval Expression library. This class allows you to:

  • Set options
  • Register type, variables, and constants
  • Execute your code
  • Compile your code
  • And more

By itself, the EvalContext is very easy to use. The most question/problem we receive is when a developer modifies the global context when he should have created an instance context, so make sure to understand this section correctly.



Contents