Dynamically construct query conditions using the LINQ Expression Tree.

Dynamically construct query conditions using the LINQ Expression Tree.

Method 1:

public static class PredicateExtensions    {        public static Expression<Func<T, bool>> True<T>() { return f => true; }        public static Expression<Func<T, bool>> False<T>() { return f => false; }        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1,            Expression<Func<T, bool>> expression2)        {            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters                    .Cast<Expression>());            return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression),            expression1.Parameters);        }        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1,               Expression<Func<T, bool>> expression2)        {            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters                 .Cast<Expression>());            return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body,                    invokedExpression), expression1.Parameters);        }    }

Method 2:

    public static class PredicateBuilder    {        public static Expression<Func<T, bool>> True<T>() { return f => true; }        public static Expression<Func<T, bool>> False<T>() { return f => false; }        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)        {            // build parameter map (from parameters of second to parameters of first)            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);            // replace parameters in the second lambda expression with parameters from the first            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);            // apply composition of lambda expression bodies to parameters from the first expression             return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);        }        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)        {            return first.Compose(second, Expression.And);        }        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)        {            return first.Compose(second, Expression.Or);        }    }

Note: In my actual application, I found that the first method of PredicateExtensions has a defect, that is, it is only suitable for DbDataContext. If it is used in EntityContext, an error is returned: LINQ to Entities does not support the node type "Invoke" of the LINQ expression ". The second method won't solve this problem. As for the root cause of the problem, I still haven't found the cause. I hope to teach with high fingers. Thank you!