Best way to concat strings and numbers in SQL server using Entity Framework 5?

问题

For some reason Microsoft decided to not support simple concat in EF5.

e.g.

Select(foo => new 
           {
             someProp = "hello" + foo.id + "/" + foo.bar
           }

This will throw if foo.id or foo.bar are numbers.

The workaround I've found is apparently this pretty peice of code:

Select(foo => new 
           {
             someProp = "hello" + 
             SqlFunctions.StringConvert((double?)foo.id).Trim()  + 
             "/" + 
             SqlFunctions.StringConvert((double?)foo.bar).Trim() 
           }

Which works fine, but is just horrid to look at.

So, is there some decent way to accomplish this with cleaner code? I'm NOT interested in doing this client side, so no .AsEnumerable() answers please.

回答1:

For those interested. I got so pissed with the lack of this feature that I implemented it myself using an ExpressionVisitor.

You can now write code like the one in the original question.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data.Objects.SqlClient;
using System.Linq;
using System.Linq.Expressions;

namespace Crawlr.Web.Code
{
    public static class ObjectSetExExtensions
    {
        public static ObjectSetEx<T> Extend<T>(this IQueryable<T> self) where T : class
        {
            return new ObjectSetEx<T>(self);
        }
    }

    public class ObjectSetEx<T> : IOrderedQueryable<T>
    {
        private readonly QueryProviderEx provider;
        private readonly IQueryable<T> source;

        public ObjectSetEx(IQueryable<T> source)
        {
            this.source = source;
            provider = new QueryProviderEx(this.source.Provider);
        }

        #region IQueryableEx<T> Members

        public IEnumerator<T> GetEnumerator()
        {
            return source.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return source.GetEnumerator();
        }

        public Type ElementType
        {
            get { return source.ElementType; }
        }

        public Expression Expression
        {
            get { return source.Expression; }
        }

        public IQueryProvider Provider
        {
            get { return provider; }
        }
        #endregion
    }

    public class QueryProviderEx : IQueryProvider
    {
        private readonly IQueryProvider source;

        public QueryProviderEx(IQueryProvider source)
        {
            this.source = source;
        }

        #region IQueryProvider Members

        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            Expression newExpression = ExpressionReWriterVisitor.Default.Visit(expression);
            IQueryable<TElement> query = source.CreateQuery<TElement>(newExpression);
            return new ObjectSetEx<TElement>(query);
        }

        public IQueryable CreateQuery(Expression expression)
        {
            Expression newExpression = ExpressionReWriterVisitor.Default.Visit(expression);
            IQueryable query = source.CreateQuery(newExpression);
            return query;
        }

        public TResult Execute<TResult>(Expression expression)
        {
            Expression newExpression = ExpressionReWriterVisitor.Default.Visit(expression);
            return source.Execute<TResult>(newExpression);
        }

        public object Execute(Expression expression)
        {
            Expression newExpression = ExpressionReWriterVisitor.Default.Visit(expression);
            return source.Execute(newExpression);
        }

        #endregion
    }

    public class ExpressionReWriterVisitor : ExpressionVisitor
    {
        public static readonly ExpressionReWriterVisitor Default = new ExpressionReWriterVisitor();

        protected override Expression VisitUnary(UnaryExpression node)
        {
            if (node.NodeType == ExpressionType.Convert && node.Operand.Type == typeof(int) && node.Type==typeof(object))
            {
                var operand = node.Operand;
                var stringConvertMethod = typeof(SqlFunctions).GetMethod("StringConvert", new Type[] { typeof(double?) });
                var trimMethod = typeof(string).GetMethod("Trim",new Type[] {});


                var dOperand = Expression.Convert(operand, typeof(double?));
                return Expression.Call(Expression.Call(stringConvertMethod, dOperand),trimMethod);
            }

            return base.VisitUnary(node);
        }      
    }
}

usage:

  var res = model
  .FooSet
  .Extend() //<- applies the magic
  .Select(foo => new 
       {
         someProp = "hello" + foo.id + "/" + foo.bar
       }

来源：https://stackoverflow.com/questions/18590880/best-way-to-concat-strings-and-numbers-in-sql-server-using-entity-framework-5