In a previous post I showed how to create expression trees to set properties on an object.
The way I did it was not very developer friendly. It involved explicitly creating
the necessary expressions because
the compiler won’t generate expression trees with property or field set expressions. Recently someone contacted me
the help develop some kind of command pattern framework that used developer friendly lambdas to generate property set expression trees. Simply putting, given this entity class: public class Person
{
public string Name { get; set; }
}
The person in question wanted to write code like this:
var et = Set((Person p) => p.Name = "me");
Where et is
the expression tree that represents
the property assignment.
So, if we can’t do this, let’s try
the next best thing that is splitting retrieving
the property information from
the retrieving
the value to assign o
the property:
var et = Set((Person p) => p.Name, () => "me");
And this is something that
the compiler can handle.
The implementation of Set receives an expression to retrieve
the property information from and another expression
the retrieve
the value to assign to
the property:
public static Expression<Action<TEntity>> Set<TEntity, TValue>(
Expression<Func<TEntity, TValue>> propertyGetExpression,
Expression<Func<TValue>> valueExpression)
The implementation of this method gets
the property information form
the body of
the property get expression (propertyGetExpression) and
the value expression (valueExpression) to build an assign expression and builds a lambda expression using
the same parameter of
the property get expression as its parameter:
public static Expression<Action<TEntity>> Set<TEntity, TValue>(
Expression<Func<TEntity, TValue>> propertyGetExpression,
Expression<Func<TValue>> valueExpression)
{
var entityParameterExpression = (ParameterExpression)(((MemberExpression)(propertyGetExpression.Body)).Expression);
return Expression.Lambda<Action<TEntity>>(
Expression.Assign(propertyGetExpression.Body, valueExpression.Body),
entityParameterExpression);
}
And now we can use
the expression to translate to another context or just compile and use it:
var et = Set((Person p) => p.Name, () => name);
Console.WriteLine(person.Name); // Prints: p => (p.Name = “me”)
var d = et.Compile();
d(person);
Console.WriteLine(person.Name); // Prints: me
It can even support closures:
var et = Set((Person p) => p.Name, () => name);
Console.WriteLine(person.Name); // Prints: p => (p.Name = value(<>c__DisplayClass0).name)
var d = et.Compile();
name = "me";
d(person);
Console.WriteLine(person.Name); // Prints: me
name = "you";
d(person);
Console.WriteLine(person.Name); // Prints: you
Not so useful in
the intended scenario (but still possible) is building an expression tree that receives
the value to assign to
the property as a parameter:
public static Expression<Action<TEntity, TValue>> Set<TEntity, TValue>(Expression<Func<TEntity, TValue>> propertyGetExpression)
{
var entityParameterExpression = (ParameterExpression)(((MemberExpression)(propertyGetExpression.Body)).Expression);
var valueParameterExpression = Expression.Parameter(typeof(TValue));
return Expression.Lambda<Action<TEntity, TValue>>(
Expression.Assign(propertyGetExpression.Body, valueParameterExpression),
entityParameterExpression,
valueParameterExpression);
}
This new expression can be used like this:
var et = Set((Person p) => p.Name);
Console.WriteLine(person.Name); // Prints: (p, Param_0) => (p.Name = Param_0)
var d = et.Compile();
d(person, "me");
Console.WriteLine(person.Name); // Prints: me
d(person, "you");
Console.WriteLine(person.Name); // Prints: you
The only caveat is that we need to be able to write code to read
the property in order to write to it.
