How to assign a value through an expression?

Question

It would be very simple if I could assign through a Lambda expression (below)

//An expression tree cannot contain an assignment operator Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName = "Tim"; 

This code above is not valid due to the assignment operator. I need to pass a lambda expression to define a property in a complex object that needs to be set. In some cases, a complex object has a List and therefore duplicates the types and names of objects, so I need a lambda to explicitly refer to a field in the updated object.

I can get the value using the following, no problem. But I'm not sure how to use the same logic to set the value, I came across Expression.Assign and I think that this may be the solution.

 Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName; var result = FindByProperty(expression); public static string FindByProperty(Expression<Func<Contract, object>> propertyRefExpr) { ComplexObj obj = new ComplexObj(); Contact myContact = new Contact(); myContact.FirstName = "Allen"; obj.Contacts = new List<Contact>{myContact}; return propertyRefExpr.Compile().Invoke(obj); } 

Update:

"passing property assignment to a method as an expression tree ..."

Using the SetValue method with ParentTypeA, the value will not work. (below code)

 Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName; obj.AssignNewValue(expression, firstName); public static void AssignNewValue(this ComplexObj obj, Expression<Func<ComplexObj, object>> expression, object value) { var propertyInfo = (PropertyInfo)((MemberExpression)expression.Body).Member; propertyInfo.SetValue(obj, value, null); } 

Answer 1

I ended up using the following solution. Greetings

 ComplexObj obj = new ComplexObj(); Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[index].FirstName; obj.AssignNewValue(expression, firstName); public static void AssignNewValue(this ComplexObj obj, Expression<Func<ComplexObj, object>> expression, object value) { ParameterExpression valueParameterExpression = Expression.Parameter(typeof(object)); Expression targetExpression = expression.Body is UnaryExpression ? ((UnaryExpression)expression.Body).Operand : expression.Body; var newValue = Expression.Parameter(expression.Body.Type); var assign = Expression.Lambda<Action<ComplexObj, object>> ( Expression.Assign(targetExpression, Expression.Convert(valueParameterExpression, targetExpression.Type)), expression.Parameters.Single(), valueParameterExpression ); assign.Compile().Invoke(obj, value); } 

Answer 2

The related question is probably the “right” answer, but just for the sake of completeness, you could do something more like this ... it improves the “tell me not ask” methodology a bit, although I can't say what I like implementation ...

 void Main() { Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].SetFirstName("Tim"); } public class ComplexObj { public ComplexObj() { Contacts = new List<SimpleObj>(); } public List<SimpleObj> Contacts {get; private set;} } public class SimpleObj { public string FirstName {get; private set;} public SimpleObj SetFirstName(string name) { this.FirstName = name; return this; } }