So I've been playing with dynamically building expression trees lately and came across this method, which seems kinda odd. At first I thought "oh cool this is exactly what I need" after constantly writing code along the lines of
var left = member is FieldInfo ? Expression.Field(instance, (FieldInfo)member) : Expression.Property(instance, (PropertyInfo)member);
var right = ...
var assign = Expression.Assign(left, right);
Yes, I know there is Expression.PropertyOrField() call, but it does roundtrip back to reflection to find member by name, where as I typically already have MemberInfo instance.
So anyway, I thought Expression.Bind() would be useful to me, but it does something I don't really understand. Given the following code:
void Main()
{
var m = typeof(Foo).GetField("Bar");
Expression.Bind(m, Expression.Constant("")).Dump();
}
public class Foo
{
public string Bar;
}
it produces MemberAssignment expression Bar = "". But there is no instance and no static reference. How would I ever apply this expression to and instance of Foo? I can't find any example of this method being used.
1 Answer
Object-Initializer expressions.
Let's say you had:
public class Foo
{
public int Property { get; set; }
}
Then you could do:
var parameter = Expression.Parameter(typeof(int), "i");
var newExpr = Expression.New(typeof(Foo));
var bindExprs = new[]
{
Expression.Bind(typeof(Foo).GetProperty("Property"), parameter)
// You can bind more properties here if you like.
};
var body = Expression.MemberInit(newExpr, bindExprs);
var lambda = Expression.Lambda<Func<int, Foo>>(body, parameter);
which is something like:
i => new Foo { Property = i }
Old:
I can't help you solve the "performance issue" you are determined to solve (will using Expression.PropertyOrField really introduce a bottleneck in your application? I'm somewhat skeptical. You should determine this before optimizing prematurely) (EDIT: Apologies for incorrectly assuming that this was a perf optimization, and as you've found out yourself, Expression.MakeMemberAccess is what you need), but I can tell you what Expression.Bind is useful for.
