如何使此查询在 LINQ to Entities 中工作？

Question

我必须遵循以下代码：

private static bool DoesColValueExist<T>(IQueryable dataToSearchIn, string colName, string colValue)
{
    int noOfClients = 1;
    Type type = typeof(T);
    if (colValue != "" && colName != "")
    {
        var property = type.GetProperty(colName);
        var parameter = Expression.Parameter(type, "p");
        var propertyAccess = Expression.MakeMemberAccess(parameter, property);
        Expression left = Expression.Call(propertyAccess, typeof(object).GetMethod("ToString", System.Type.EmptyTypes));
        left = Expression.Call(left, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
        Expression right = Expression.Constant(colValue.ToLower(), typeof(string));
        MethodInfo method = typeof(string).GetMethod("Equals", new[] { typeof(string) });
        Expression searchExpression = Expression.Call(left, method, right);


        MethodCallExpression whereCallExpression = Expression.Call(
            typeof(Queryable),
            "Where",
            new Type[] { type },
            dataToSearchIn.Expression,
            Expression.Lambda<Func<T, bool>>(searchExpression, new ParameterExpression[] { parameter }));
        var searchedData = dataToSearchIn.Provider.CreateQuery(whereCallExpression);
        noOfClients = searchedData.Cast<T>().Count();

        if (noOfClients == 0)
            return false;
        else
            return true;
    }
    return true;
}

它适用于 LINQ to SQL，但使用 LINQ to Entities，我收到错误：

LINQ to Entities 无法识别方法“System.String ToString()”方法，并且该方法无法转换为存储表达式。

Answer 1

Linq to Entities 不支持 .ToString() 方法。我也不确定对非字符串类型使用字符串比较是否是个好主意。但并非全部丢失。我想出了以下解决方案：

public partial class MyEntity
{
    public int ID { get; set; }
    public int Type { get; set; }
    public string X { get; set; }
}

public class MyContext : DbContext
{
    public DbSet<MyEntity> Entities { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        Database.SetInitializer(new DropCreateDatabaseIfModelChanges<MyContext>());

        using (var ctx = new MyContext())
        {
            if (!ctx.Entities.Any())
            {
                ctx.Entities.Add(new MyEntity() { ID = 1, Type = 2, X = "ABC" });
                ctx.SaveChanges();
            }

            Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.X, "aBc"));
            Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.X, "aBcD"));
            Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.Type, 2));
            Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.Type, 5));

        }
    }

    private static bool DoesColValueExist<TEntity, TProperty>(IQueryable<TEntity> dataToSearchIn, Expression<Func<TEntity, TProperty>> property, TProperty colValue)
    {

        var memberExpression = property.Body as MemberExpression;
        if (memberExpression == null || !(memberExpression.Member is PropertyInfo))
        {
            throw new ArgumentException("Property expected", "property");
        }

        Expression left = property.Body;
        Expression right = Expression.Constant(colValue, typeof(TProperty));
        if (typeof(TProperty) == typeof(string))
        {
            MethodInfo toLower = typeof(string).GetMethod("ToLower", new Type[0]);
            left = Expression.Call(left, toLower);
            right = Expression.Call(right, toLower);
        }

        Expression searchExpression = Expression.Equal(left, right);
        var lambda = Expression.Lambda<Func<TEntity, bool>>(Expression.Equal(left, right), new ParameterExpression[] { property.Parameters.Single() });

        return dataToSearchIn.Where(lambda).Any();                
    }
}

它的好处是它比基于字符串的解决方案更安全 - 参数的值必须与属性的值相同。该属性又必须是作为第一个参数传递的 IQueryable'1 的通用类型的实体的成员。另一件有用的事情是，当您开始为第二个参数输入 lambda 表达式时，针对此方法进行编码时，intellisense 将向您显示实体的成员。在方法本身中，当我对属性值和请求的值调用 .ToLower() 以使比较不区分大小写时，我添加了字符串类型的异常。对于非字符串类型，值按“原样”进行比较，即不进行任何修改。 上面的示例是完整的 - 您可以将其复制并粘贴到控制台应用程序项目中（尽管您需要引用 EntityFramework.dll）。 希望这可以帮助。

Answer 2

试试这个：

private static bool DoesColValueExist<T>(IQueryable dataToSearchIn, string colName, string colValue)
{
    int noOfClients = 1;
    Type type = typeof(T);
    if (colValue != "" && colName != "")
    {
        var property = type.GetProperty(colName);
        var parameter = Expression.Parameter(type, "p");
        var propertyAccess = Expression.MakeMemberAccess(parameter, property);
        Expression left = property.PropertyType == typeof(string) ? propertyAccess : Expression.Call(propertyAccess, typeof(object).GetMethod("ToString", System.Type.EmptyTypes));
        left = Expression.Call(left, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
        Expression right = Expression.Constant(colValue.ToLower(), typeof(string));
        MethodInfo method = typeof(string).GetMethod("Equals", new[] { typeof(string) });
        Expression searchExpression = Expression.Call(left, method, right);


        MethodCallExpression whereCallExpression = Expression.Call(
            typeof(Queryable),
            "Where",
            new Type[] { type },
            dataToSearchIn.Expression,
            Expression.Lambda<Func<T, bool>>(searchExpression, new ParameterExpression[] { parameter }));
        var searchedData = dataToSearchIn.Provider.CreateQuery(whereCallExpression);
        noOfClients = searchedData.Cast<T>().Count();

        if (noOfClients == 0)
            return false;
        else
            return true;
    }
    return true;
}

基本上，如果属性是字符串，那么它不会调用ToString() 方法。

希望对你有帮助。