Generic Expression tree with 'OR' clause for each supplied property

问题

I have created a generic search extension method for IQueryable that enables you to search for a single property to see if a search term is contained within it.

http://jnye.co/Posts/6/c%23-generic-search-extension-method-for-iqueryable

I now want to enable the user to select multiple properties to search within each, matching if any property contains the text.

The code:

The user enters the following code to perform this search:

string searchTerm = "Essex";
context.Clubs.Search(searchTerm, club => club.Name, club => club.County)

//Note: If possible I would rather something closer to the following syntax...
context.Clubs.Search(club => new[]{ club.Name, club.County}, searchTerm);
// ... or, even better, something similar to this...
context.Clubs.Search(club => new { club.Name, club.County}, searchTerm);

This will return any golf club with 'Essex' in the Name or as the County.

    public static IQueryable<TSource> Search<TSource>(this IQueryable<TSource> source, string searchTerm, params Expression<Func<TSource, string>>[] stringProperties)
    {
        if (String.IsNullOrEmpty(searchTerm))
        {
            return source;
        }

        // The lamda I would like to reproduce:
        // source.Where(x => x.[property1].Contains(searchTerm)
        //                || x.[property2].Contains(searchTerm)
        //                || x.[property3].Contains(searchTerm)...)

        //Create expression to represent x.[property1].Contains(searchTerm)
        var searchTermExpression = Expression.Constant(searchTerm);


        //Build parameters
        var parameters = stringProperties.SelectMany(prop => prop.Parameters);
        Expression orExpression = null;

        //Build a contains expression for each property
        foreach (var stringProperty in stringProperties)
        {
            var checkContainsExpression = Expression.Call(stringProperty.Body, typeof(string).GetMethod("Contains"), searchTermExpression);
            if (orExpression == null)
            {
                orExpression = checkContainsExpression;
            }

            //Build or expression for each property
            orExpression = Expression.OrElse(orExpression, checkContainsExpression);
        }

        var methodCallExpression = Expression.Call(typeof(Queryable),
                                                   "Where",
                                                   new Type[] { source.ElementType },
                                                   source.Expression,
                                                   Expression.Lambda<Func<TSource, bool>>(orExpression, parameters));

        return source.Provider.CreateQuery<TSource>(methodCallExpression);
    }

The error

If I change the number of parameters supplied to 1:

Expression.Lambda<Func<TSource, bool>>(orExpression, parameters.First()));

I get a new error:

UPDATE

I have written a post on the work discussed in this question. Check it out on GitHub too.

回答1:

Here we go; you were pretty close - as I noted in comments, the key piece here is to use ExpressionVisitor to re-write the trees in terms of the single parameter you want to keep:

using System;
using System.Linq;
using System.Linq.Expressions;
static class Program
{
    static void Main()
    {
        var data = new[] { new Foo { A = "x1", B = "y1", C = "y1" }, new Foo { A = "y2", B = "y2", C = "y2" },
            new Foo { A = "y3", B = "y3", C = "x3" } }.AsQueryable();

        var result = data.Search("x", x => x.A, x => x.B, x => x.C);

        foreach (var row in result)
        {
            Console.WriteLine("{0}, {1}, {2}", row.A, row.B, row.C);
        }
    }
    class Foo
    {
        public string A { get; set; }
        public string B { get; set; }
        public string C { get; set; }
    }
    public class SwapVisitor : ExpressionVisitor
    {
        private readonly Expression from, to;
        public SwapVisitor(Expression from, Expression to)
        {
            this.from = from;
            this.to = to;
        }
        public override Expression Visit(Expression node)
        {
            return node == from ? to : base.Visit(node);
        }
        public static Expression Swap(Expression body, Expression from, Expression to)
        {
            return new SwapVisitor(from, to).Visit(body);
        }
    }
    public static IQueryable<TSource> Search<TSource>(this IQueryable<TSource> source, string searchTerm, params Expression<Func<TSource, string>>[] stringProperties)
    {
        if (String.IsNullOrEmpty(searchTerm))
        {
            return source;
        }
        if (stringProperties.Length == 0) return source.Where(x => false);


        // The lamda I would like to reproduce:
        // source.Where(x => x.[property1].Contains(searchTerm)
        //                || x.[property2].Contains(searchTerm)
        //                || x.[property3].Contains(searchTerm)...)

        //Create expression to represent x.[property1].Contains(searchTerm)
        var searchTermExpression = Expression.Constant(searchTerm);


        var param = stringProperties[0].Parameters.Single();
        Expression orExpression = null;

        //Build a contains expression for each property
        foreach (var stringProperty in stringProperties)
        {
            // re-write the property using the param we want to keep
            var body = SwapVisitor.Swap(stringProperty.Body, stringProperty.Parameters.Single(), param);

            var checkContainsExpression = Expression.Call(
                body, typeof(string).GetMethod("Contains"), searchTermExpression);

            if (orExpression == null)
            {
                orExpression = checkContainsExpression;
            }
            else
            {   // compose
                orExpression = Expression.OrElse(orExpression, checkContainsExpression);
            }
        }

        var lambda = Expression.Lambda<Func<TSource, bool>>(orExpression, param);
        return source.Where(lambda);
    }
}

来源：https://stackoverflow.com/questions/15558456/generic-expression-tree-with-or-clause-for-each-supplied-property