C# Eval Expression My First Compilation

In this tutorial, we will assume that you have read and understood the concept we introduced in the My First Evaluation tutorial. If you have not read it, make sure you start with this previous tutorial, as we might skip the explanation of a few concepts.

So what is the major difference between executing and compiling a dynamic code?

  • The Compile method will compile the expression into a method
  • The Execute method call the Compile method under the hood and then execute the returned compiled method.

When executing the same expression with the Execute method multiple times, it is fast as we retrieve the compiled method from the cache. However, it still takes some time, as we need to create a cache key that has a performance cost. So when you need to execute multiple times the same expression, you should store the compiled method in a local or static variable.

How much faster? Using directly the compiled method returned from the Compile can be 1000x more quickly than using the Execute method. It surely doesn't make any difference if you are only looping on 10 items, as both ways, it will take nearly 0ms. However, if you need to evaluate millions of items, it makes a massive difference between a few milliseconds and seconds.

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In this tutorial, you will learn how to use our library to:

Compile a C# expression

Like the Execute method, the Compile method takes as the first parameter the expression to compile.

In this example, we will dynamically create a list of int and filter it using LINQ to return only items greater than 2. The expression will be compiled into a method and then executed. The Func<List<int>> return type will be explained in the next example.

// NOTE: The returned list contains "3" and "4"
var compiled = Eval.Compile<Func<List<int>>>(@"
var list = new List<int>() { 1, 2, 3, 4 };
return list.Where(x => x > 2).ToList();
");

var list = compiled();

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Compile a C# expression with return type

In the last example, we successfully compiled an expression, but what exactly was the Func<List<int>> for?

The Func<List<int>> means that the Compile method should create a method that takes no parameter and returns a List<int>. Usually, the Compile method takes either an Action or Func.

  • An Action should be used when the method doesn't return any type (like when you create a void method).
  • A Func should be used when the compiled method should return a specific type

You can learn more about Action and Func on the official Microsoft documentation and Google.

We will re-use the same previous example as we now understand how you can specify a return type to the created method.

// NOTE: The returned list contains "3" and "4"
var compiled = Eval.Compile<Func<List<int>>>(@"
var list = new List<int>() { 1, 2, 3, 4 };
return list.Where(x => x > 2).ToList();
");

var list = compiled();

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Compile a C# expression with variable names

With the 2 previous topics, you should now be able to compile an expression and understand how to return the type of the compiled method.

However, almost every time, the compiled method would also have parameters.

There are multiple ways to pass variable names, such as:

  • Position
  • Named variables
  • ExpandoObject

In the following example, we will see all the different ways to pass variable names that will be used in our expression to compile.

var list = new List<int>() { 1, 2, 3, 4 };
var greaterThan = 2;

// Passing parameters with position
// NOTE: In this example, {0} is the first `List<int>` and {1} is the `int`
{
	var compiled = Eval.Compile<Func<List<int>, int, List<int>>>("{0}.Where(x => x > {1})");
	var rList = compiled(list, greaterThan);
	FiddleHelper.WriteTable((IEnumerable<int>)rList);
}

// Passing parameters with named variables
// NOTE: You need to name as many variables as the `Func` have (without counting the return type)
{
	var compiled = Eval.Compile<Func<List<int>, int, List<int>>>("list.Where(x => x > greaterThan)", "list", "greaterThan");
	var rList = compiled(list, greaterThan);
	FiddleHelper.WriteTable((IEnumerable<int>)rList);
}

// Passing parameters with an ExpandoObject
// NOTE: More flexible than the 2 previous examples but less robust as the library assume any missing name is part of the ExpandoObject
// NOTE: We had to cast the list to `(List<int>)` as until the method is executed, the library doesn't know which type is the variable `list`
{
	dynamic expandoObject = new ExpandoObject();
	expandoObject.list = list;
	expandoObject.greaterThan= greaterThan;
	
	var compiled = Eval.Compile<Func<ExpandoObject, List<int>>>("((List<int>)list).Where(x => x > greaterThan)");
	var rList = compiled(expandoObject);
	FiddleHelper.WriteTable((IEnumerable<int>)rList);
}

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Compile a C# expression from a string

Our library added syntactic sugar to extend the string type with the Execute method. As you can expect, our library also extends the string type with the Compile method.

There is no difference here between calling Eval.Compile and "the_string_expression".Compile. Both call the same method under the hood.

In this example, we will compile an expression and loop to show only items in our list that currently have a higher value than the current position in our loop.

var list = new List<int>() { 1, 2, 3, 4 };

var expression = "list.Where(x => x > i)";

var compiled = expression.Compile<Func<List<int>, int, List<int>>>("list", "i");
// SIMILAR TO: list = Eval.Compile<Func<List<int>, int, List<int>>>(expression, "list", "i");		

for(int i = 0; i < 4; i++)
{
	var rList = compiled(list, i);
	FiddleHelper.WriteTable("i=" + i, rList);
}

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Compile a C# expression from a context

All the Compile methods we learned in previous examples use the global EvalContext. You can learn more about why and when you should use an instance context in the EvalContext documentation.

In this example, we will create an instance context, add a new extension method named GreaterThan and use the Compile method. Then after we will loop to show only item in our list that currently have a higher value than the current position in our loop.

var list = new List<int>() { 1, 2, 3, 4 };		
var context = new EvalContext();

context.AddMethod(@"
bool GreaterThan(this int x, int y)
{
	return x > y;
}
");

var compiled = context.Compile<Func<List<int>, int, List<int>>>("list.Where(x => x.GreaterThan(i))", "list", "i");	

for(int i = 0; i < 4; i++)
{
	var rList = compiled(list, i);
	FiddleHelper.WriteTable("i=" + i, rList);
}

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The method GreaterThan only exists for this context. That means the Eval.Compile is unaware and cannot use this method.

Conclusion

In this getting started tutorial, you learned how to use the Compile method with an expression to return a compiled method.

Using the Compile method is a little bit more complex than the Execute method to understand. However, make sure you take the time to really learn it, as this is an essential method to master to make sure you take advantage of the performance benefit this method offers.


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