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copied!<p>It was difficult for me to understand hvd's <a href="https://stackoverflow.com/questions/9231569/exception-using-orelse-and-andalso-expression-methods?answertab=active#tab-top/" title="hvds answer">answer</a> so I created some code to explain it in a different way. hvd should get the credit for suggesting the ExpressionVisitor. I just couldn't understand the example in the context of Linq to X type input functions I was using.</p> <p>I hope this helps somebody else coming to the question from that perspective.</p> <p>Also, I created the combining code as extension methods to make it a little easier to use.</p> <hr> <pre><code>using System; using System.Collections.Generic; using System.Linq.Expressions; namespace ConsoleApplication3 { class Program { static void Main(string[] args) { var combined = TryCombiningExpressions(c => c.FirstName == "Dog", c => c.LastName == "Boy"); Console.WriteLine("Dog Boy should be true: {0}", combined(new FullName { FirstName = "Dog", LastName = "Boy" })); Console.WriteLine("Cat Boy should be false: {0}", combined(new FullName { FirstName = "Cat", LastName = "Boy" })); Console.ReadLine(); } public class FullName { public string FirstName { get; set; } public string LastName { get; set; } } public static Func<FullName, bool> TryCombiningExpressions(Expression<Func<FullName, bool>> func1, Expression<Func<FullName, bool>> func2) { return func1.CombineWithAndAlso(func2).Compile(); } } public static class CombineExpressions { public static Expression<Func<TInput, bool>> CombineWithAndAlso<TInput>(this Expression<Func<TInput, bool>> func1, Expression<Func<TInput, bool>> func2) { return Expression.Lambda<Func<TInput, bool>>( Expression.AndAlso( func1.Body, new ExpressionParameterReplacer(func2.Parameters, func1.Parameters).Visit(func2.Body)), func1.Parameters); } public static Expression<Func<TInput, bool>> CombineWithOrElse<TInput>(this Expression<Func<TInput, bool>> func1, Expression<Func<TInput, bool>> func2) { return Expression.Lambda<Func<TInput, bool>>( Expression.AndAlso( func1.Body, new ExpressionParameterReplacer(func2.Parameters, func1.Parameters).Visit(func2.Body)), func1.Parameters); } private class ExpressionParameterReplacer : ExpressionVisitor { public ExpressionParameterReplacer(IList<ParameterExpression> fromParameters, IList<ParameterExpression> toParameters) { ParameterReplacements = new Dictionary<ParameterExpression, ParameterExpression>(); for (int i = 0; i != fromParameters.Count && i != toParameters.Count; i++) ParameterReplacements.Add(fromParameters[i], toParameters[i]); } private IDictionary<ParameterExpression, ParameterExpression> ParameterReplacements { get; set; } protected override Expression VisitParameter(ParameterExpression node) { ParameterExpression replacement; if (ParameterReplacements.TryGetValue(node, out replacement)) node = replacement; return base.VisitParameter(node); } } } } </code></pre>

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