How do I dynamically create an Expression predicate from Expression?

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It’s hard to mix compiler-generated expression trees and hand-made ones, precisely because of this sort of thing – extracting out the ParameterExpressions is tricky. So let’s start from scratch:

ParameterExpression argParam = Expression.Parameter(typeof(Service), "s");
Expression nameProperty = Expression.Property(argParam, "Name");
Expression namespaceProperty = Expression.Property(argParam, "Namespace");

var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");

Expression e1 = Expression.Equal(nameProperty, val1);
Expression e2 = Expression.Equal(namespaceProperty, val2);
var andExp = Expression.AndAlso(e1, e2);

var lambda = Expression.Lambda<Func<Service, bool>>(andExp, argParam);

One important aspect I’ve changed is the type passed to Expression.Parameter – it certainly looks like it should be a Service rather than a string.

I’ve given that a try, and it seemed to work when I called lambda.Compile and executed it on a couple of sample Service objects…

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