What you’re looking for is the ability to compose expressions, just as you can compose functions:
public static Expression<Func<T, TResult>> Compose<T, TIntermediate, TResult>(
this Expression<Func<T, TIntermediate>> first,
Expression<Func<TIntermediate, TResult>> second)
{
return Expression.Lambda<Func<T, TResult>>(
second.Body.Replace(second.Parameters[0], first.Body),
first.Parameters[0]);
}
This relies on the following method to replace all instances of one expression with another:
public class ReplaceVisitor:ExpressionVisitor
{
private readonly Expression from, to;
public ReplaceVisitor(Expression from, Expression to)
{
this.from = from;
this.to = to;
}
public override Expression Visit(Expression ex)
{
if(ex == from) return to;
else return base.Visit(ex);
}
}
public static Expression Replace(this Expression ex,
Expression from,
Expression to)
{
return new ReplaceVisitor(from, to).Visit(ex);
}
You can now take an expression selecting a property:
Expression<Func<Customer, object>> propertySelector = cust => cust.Name;
And an expression selecting that object from the model:
Expression<Func<CustomerModel, Customer>> modelSelector = model => model.Customer;
and compose them:
Expression<Func<Customer, object> magic = modelSelector.Compose(propertySelector);