C# Language Expression Trees Understanding the expressions API
Example
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.
- Check if the product is taxable
- If it is, multiply the line total by the applicable tax rate and return that amount
- 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);
