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POHow do I compose Linq Expressions? ie Func<Exp<Func<X, Y>>, Exp<Func<Y, Z>>, Exp<Func<X, Z>>>
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copied!<p>I'm creating a <code>Validator<T></code> class. I'm attempting to implement the Linq <code>SelectMany</code> extension methods for my validator to be able to compose expressions using a Linq query and validate the final result even when the underlying values change.</p> <p>The following test code demonstrates my intent.</p> <pre><code>var a = 2; var b = 3; var va = Validator.Create(() => a, n => n >= 0 && n < 5); var vb = Validator.Create(() => b, n => n >= 0 && n < 5); var vc = from ia in va from ib in vb select ia + ib; Debug.Assert(vc.Value == a + b); //2 + 3 Debug.Assert(vc.Value == 5); Debug.Assert(vc.IsValid == true); a = 7; Debug.Assert(vc.Value == a + b); //7 + 3 Debug.Assert(vc.Value == 10); Debug.Assert(va.IsValid == false); Debug.Assert(vb.IsValid == true); Debug.Assert(vc.IsValid == false); </code></pre> <p>I've seen the following question <a href="https://stackoverflow.com/questions/53597/how-do-i-compose-existing-linq-expressions">How do I compose existing Linq Expressions</a> which shows me how to compose two <code>Func<T, bool></code>'s together using an <code>And</code> expression, but I need to be able to compose functions together in a more, well, functional way.</p> <p>I have, for example, the following two expressions:</p> <pre><code>public Expression<Func<T>> ValueExpression { get; private set; } public Expression<Func<T, bool>> ValidationExpression { get; private set; } </code></pre> <p>I wish to create a new expression like this:</p> <pre><code> public Expression<Func<bool>> IsValidExpression { get { // TODO: Compose expressions rather than compile & invoke. } } </code></pre> <p>More succinctly I'm trying to create these functions:</p> <pre><code>// Specific case Func<Expression<Func<T>>, Expression<Func<T, bool>>, Expression<Func<bool>>> // General case Func<Expression<Func<X, Y>>, Expression<Func<Y, Z>>, Expression<Func<X, Z>>> </code></pre> <p>The general case function can be modified to accept different numbers of generic arguments as needed to compose any function.</p> <p>I've searched Stack Overflow (of course) and the web, but haven't an example that solves this issue.</p> <p>My code for the <code>Validator<T></code> class is below.</p> <pre><code>public class Validator<T> { public Validator(Expression<Func<T>> valueFunc, Expression<Func<T, bool>> validationFunc) { this.ValueExpression = valueFunc; this.ValidationExpression = validationFunc; } public Expression<Func<T>> ValueExpression { get; private set; } public Expression<Func<T, bool>> ValidationExpression { get; private set; } public T Value { get { return this.ValueExpression.Compile().Invoke(); } } public bool IsValid { get { return this.IsValidExpression.Compile().Invoke(); } } public Expression<Func<bool>> IsValidExpression { get { // TODO: Compose expressions. } } } </code></pre> <p>My <code>SelectMany</code> extensions contain loads of yucky <code>.Compile().Invoke()</code> which I want to get rid of.</p> <pre><code>public static Validator<U> SelectMany<T, U>(this Validator<T> @this, Expression<Func<T, Validator<U>>> k) { Expression<Func<T>> fvtv = @this.ValueExpression; Expression<Func<Validator<U>>> fvu = () => k.Compile().Invoke(fvtv.Compile().Invoke()); Expression<Func<U>> fvuv = fvu.Compile().Invoke().ValueExpression; Expression<Func<U, bool>> fvtiv = u => @this.ValidationExpression.Compile().Invoke(fvtv.Compile().Invoke()); return fvuv.ToValidator(fvtiv); } public static Validator<V> SelectMany<T, U, V>(this Validator<T> @this, Expression<Func<T, Validator<U>>> k, Expression<Func<T, U, V>> s) { Expression<Func<Validator<U>>> fvu = () => @this.SelectMany(k); Expression<Func<T>> fvtv = @this.ValueExpression; Expression<Func<U>> fvuv = fvu.Compile().Invoke().ValueExpression; Expression<Func<T, bool>> fvtiv = @this.ValidationExpression; Expression<Func<U, bool>> fvuiv = u => fvu.Compile().Invoke().ValidationExpression.Compile().Invoke(u); Expression<Func<V>> fvv = () => s.Compile().Invoke(fvtv.Compile().Invoke(), fvuv.Compile().Invoke()); Expression<Func<V, bool>> fvviv = v => fvtiv.Compile().Invoke(fvtv.Compile().Invoke()) && fvuiv.Compile().Invoke(fvuv.Compile().Invoke()); return fvv.ToValidator(fvviv); } </code></pre> <p>Thanks in advance!</p>

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