What will we be building today? How about the beginnings a validation-helper for ensuring that a field, property, or argument is not null? How often have you written the following code?
Now this code is all good and well. But at some point we might change the name of the parameter but forget to update the string that represents the name of the parameter. Instead we can use an Expression which will dynamically get the name of the member which guarantees that the argument name provided to the ArgumentNullException constructor is always correct. So instead of using a string to represent the argument name, let's use an Expression:
Nice and simple! Now for some Expression basics.
Expression Basics
As seen on msdn the System.Linq.Expression namespace can be explained as follows:The System.Linq.Expressions namespace contains classes, interfaces and enumerations that enable language-level code expressions to be represented as objects in the form of expression trees.
The abstract class Expression provides the root of a class hierarchy used to model expression trees.
The classes in this namespace that derive from Expression, for example MemberExpression and ParameterExpression, are used to represent nodes in an expression tree. The Expression class contains static (Shared in Visual Basic) factory methods to create expression tree nodes of the various types.
The enumeration type ExpressionType specifies the unique node types.
I think that's enough of that for now. Let's dive into some code.
Following is what our method might look like without utilizing Expressions.
As we will be passing properties, fields, and method parameters into our IsNotNull method, we will be dealing exclusively with Expressions of type MemberExpression.
The best tool in the Expression toolbox is the ExpressionVisitor. In a nutshell an ExpressionVisitor handles the navigation and of an ExpressionTree (which we will talk about in more detail in my next post).
To handle extracting the MemberExpressions from our ExpressionTree we will create an ExpressionVisitor and override the VisitMember method. The VisitMember method will simply add the MemberExpression to a SortedList
Now that we have a method of extracting a MemberExpression, we can implement our IsNotNull
- Extract the MemberExpression
- Extract the value of the property, field, or argument from the MemberExpression
- Check if the value is null
- If null, throw an ArgumentNullException
Here is the finished code for the Validate class:
And finally here is an example of how to invoke IsNotNull:
Source code can be downloaded from github here.
