Following is most basic expression tree that is created by lambda.

Expression<Func<int, bool>> lambda = num => num == 42;

To create expression trees 'by hand', one should use Expression class.

Expression above would be equivalent to:

ParameterExpression parameter = Expression.Parameter(typeof(int), "num"); // num argument
ConstantExpression constant = Expression.Constant(42, typeof(int)); // 42 constant
BinaryExpression equality = Expression.Equals(parameter, constant); // equality of two expressions (num == 42)
Expression<Func<int, bool>> lambda = Expression.Lambda<Func<int, bool>>(equality, parameter);

Expression trees represent code in a tree-like data structure, where each node is an expression

Expression Trees enables dynamic modification of executable code, the execution of LINQ queries in various databases, and the creation of dynamic queries. You can compile and run code represented by expression trees.

These are also used in the dynamic language run-time (DLR) to provide interoperability between dynamic languages and the .NET Framework and to enable compiler writers to emit expression trees instead of Microsoft intermediate language (MSIL).

Expression Trees can be created Via

1. Anonymous lambda expression,
2. Manually by using the System.Linq.Expressions namespace.

Expression Trees from Lambda Expressions

When a lambda expression is assigned to Expression type variable , the compiler emits code to build an expression tree that represents the lambda expression.

The following code examples shows how to have the C# compiler create an expression tree that represents the lambda expression num => num < 5.

Expression<Func<int, bool>> lambda = num => num < 5;

Expression Trees by Using the API

Expression Trees also created using the Expression Class. This class contains static factory methods that create expression tree nodes of specific types.

Below are few type of Tree nodes.

1. ParameterExpression
2. MethodCallExpression

The following code example shows how to create an expression tree that represents the lambda expression num => num < 5 by using the API.

ParameterExpression numParam = Expression.Parameter(typeof(int), "num");
ConstantExpression five = Expression.Constant(5, typeof(int));
BinaryExpression numLessThanFive = Expression.LessThan(numParam, five);
Expression<Func<int, bool>> lambda1 = Expression.Lambda<Func<int, bool>>(numLessThanFive,new ParameterExpression[] { numParam });

Define a new visitor class by overriding some of the methods of ExpressionVisitor (opens new window):

class PrintingVisitor : ExpressionVisitor {
    protected override Expression VisitConstant(ConstantExpression node) {
        Console.WriteLine("Constant: {0}", node);
        return base.VisitConstant(node);
    }
    protected override Expression VisitParameter(ParameterExpression node) {
        Console.WriteLine("Parameter: {0}", node);
        return base.VisitParameter(node);
    }
    protected override Expression VisitBinary(BinaryExpression node) {
        Console.WriteLine("Binary with operator {0}", node.NodeType);
        return base.VisitBinary(node);
    }
}

Call Visit to use this visitor on an existing expression:

Expression<Func<int,bool>> isBig = a => a > 1000000;
var visitor = new PrintingVisitor();
visitor.Visit(isBig);

We're going to use the expression tree API to create a CalculateSalesTax tree. In plain English, here's a summary of the steps it takes to create the tree.

1. Check if the product is taxable
2. If it is, multiply the line total by the applicable tax rate and return that amount
3. Otherwise return 0

//For reference, we're using the API to build this lambda expression
    orderLine => orderLine.IsTaxable ? orderLine.Total * orderLine.Order.TaxRate : 0;

//The orderLine parameter we pass in to the method.  We specify it's type (OrderLine) and the name of the parameter.
    ParameterExpression orderLine = Expression.Parameter(typeof(OrderLine), "orderLine");

//Check if the parameter is taxable;  First we need to access the is taxable property, then check if it's true
    PropertyInfo isTaxableAccessor = typeof(OrderLine).GetProperty("IsTaxable");
    MemberExpression getIsTaxable = Expression.MakeMemberAccess(orderLine, isTaxableAccessor);
    UnaryExpression isLineTaxable = Expression.IsTrue(getIsTaxable);

//Before creating the if, we need to create the braches
    //If the line is taxable, we'll return the total times the tax rate; get the total and tax rate, then multiply
    //Get the total
    PropertyInfo totalAccessor = typeof(OrderLine).GetProperty("Total");
    MemberExpression getTotal = Expression.MakeMemberAccess(orderLine, totalAccessor);
    
    //Get the order
    PropertyInfo orderAccessor = typeof(OrderLine).GetProperty("Order");
    MemberExpression getOrder = Expression.MakeMemberAccess(orderLine, orderAccessor);
    
    //Get the tax rate - notice that we pass the getOrder expression directly to the member access
    PropertyInfo taxRateAccessor = typeof(Order).GetProperty("TaxRate");
    MemberExpression getTaxRate = Expression.MakeMemberAccess(getOrder, taxRateAccessor);
    
    //Multiply the two - notice we pass the two operand expressions directly to multiply
    BinaryExpression multiplyTotalByRate = Expression.Multiply(getTotal, getTaxRate);
    
//If the line is not taxable, we'll return a constant value - 0.0 (decimal)
    ConstantExpression zero = Expression.Constant(0M);

//Create the actual if check and branches
    ConditionalExpression ifTaxableTernary = Expression.Condition(isLineTaxable, multiplyTotalByRate, zero);
    
//Wrap the whole thing up in a "method" - a LambdaExpression
    Expression<Func<OrderLine, decimal>> method = Expression.Lambda<Func<OrderLine, decimal>>(ifTaxableTernary, orderLine);

Syntax

• Expression name = lambdaExpression;

Parameters

Remarks

Intro to Expression Trees

Where we came from

Expression trees are all about consuming "source code" at runtime. Consider a method which calculates the sales tax due on a sales order decimal CalculateTotalTaxDue(SalesOrder order). Using that method in a .NET program is easy — you just call it decimal taxDue = CalculateTotalTaxDue(order);. What if you want to apply it to all the results from a remote query (SQL, XML, a remote server, etc)? Those remote query sources cannot call the method! Traditionally, you would have to invert the flow in all these cases. Make the entire query, store it in memory, then loop through the results and calculate tax for each result.

How to avoid flow inversion's memory and latency problems

Expression trees are data structures in a format of a tree, where each node holds an expression. They are used to translate the compiled instructions (like methods used to filter data) in expressions which could be used outside of the program environment such as inside a database query.

The problem here is that a remote query cannot access our method. We could avoid this problem if instead, we sent the instructions for the method to the remote query. In our CalculateTotalTaxDue example, that means we send this information:

1. Create a variable to store the total tax
2. Loop through all the lines on the order
3. For each line, check if the product is taxable
4. If it is, multiply the line total by the applicable tax rate and add that amount to the total
5. Otherwise do nothing

With those instructions, the remote query can perform the work as it's creating the data.

There are two challenges to implementing this. How do you transform a compiled .NET method into a list of instructions, and how do you format the instructions in a way that they can be consumed by the remote system?

Without expression trees, you could only solve the first problem with MSIL. (MSIL is the assembler-like code created by the .NET compiler.) Parsing MSIL is possible, but it's not easy. Even when you do parse it properly, it can be hard to determine what the original programmer's intent was with a particular routine.

Expression trees save the day

Creating expression trees

Lambda expressions cannot create every type of expression tree. In those cases, you can use the Expressions API manually to create the tree you need to. In the Understanding the expressions API example, we create the CalculateTotalSalesTax expression using the API.

NOTE: The names get a bit confusing here. A lambda expression (two words, lower case) refers to the block of code with a => indicator. It represents an anonymous method in C# and is converted into either a Delegate or Expression. A LambdaExpression (one word, PascalCase) refers to the node type within the Expression API which represents a method you can execute.

Expression Trees and LINQ

One of the most common uses of expression trees is with LINQ and database queries. LINQ pairs an expression tree with a query provider to apply your instructions to the target remote query. For example, the LINQ to Entity Framework query provider transforms an expression tree into SQL which is executed against the database directly.

Putting all the pieces together, you can see the real power behind LINQ.

1. Write a query using a lambda expression: products.Where(x => x.Cost > 5)
2. The compiler transforms that expression into an expression tree with the instructions "check if the Cost property of the parameter is greater than five".
3. The query provider parses the expression tree and produces a valid SQL query SELECT * FROM products WHERE Cost > 5
4. The ORM projects all the results into POCOs and you get a list of objects back

Notes

• Expression trees are immutable. If you want to change an expression tree you need to create a new one, copy the existing one into the new one (to traverse an expression tree you can use the ExpressionVisitor) and make the wanted changes.