Question

I am trying to convert a DateTime to a String before calling contains on it. However despite my efforts of putting the result of one expression into another I fail miserably.

The code is derived from the highest answer to this question jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ — but needs dynamic operators.

Assume I have the following method as StringExtension from the question:

public static class StringExtensions
{
  public static MemberExpression ToMemberExpression(this string source, ParameterExpression p)
  {
    if (p == null)
        throw new ArgumentNullException("p");

    string[] properties = source.Split('.');

    Expression expression = p;
    Type type = p.Type;

    foreach (var prop in properties)
    {
        var property = type.GetProperty(prop);
        if (property == null)
            throw new ArgumentException("Invalid expression", "source");

        expression = Expression.MakeMemberAccess(expression, property);
        type = property.PropertyType;
    }

    return (MemberExpression)expression;
  }
}

Therefore I have the following method also from the question which I have then adapted for DateTime.

public virtual Expression<Func<T, bool>> CreateExpression<T>(string searchField, string searchString, string searchOper)
    {
        Expression exp = null;
        var p = Expression.Parameter(typeof(T), "p");

        Expression propertyAccess = searchField.ToMemberExpression(p);

        switch (searchOper)
        {
            case "bw":
                exp = Expression.Call(propertyAccess, typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) }), Expression.Constant(searchString));
                break;

            // New code by me
            case "cn":

                if (propertyAccess.Type == typeof(DateTime))
                {
                    // My faulty logic - from Jon Skeet answer below

                    Expression toStringCall = Expression.Call(
                        propertyAccess, "ToString",
                        null,
                        new[] { Expression.Constant("D") });

                    Expression containsCall = Expression.Call(
                        toStringCall, "Contains",
                        null,
                        new[] { Expression.Constant(searchString) });

                    exp = containsCall;
                }
                else
                {
                    // Unchanged
                    exp = Expression.Call(propertyAccess, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), Expression.Constant(searchString));
                }
                break;
            case "ew":
                exp = Expression.Call(propertyAccess, typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) }), Expression.Constant(searchString));
                break;
            case "gt":
                exp = Expression.GreaterThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                break;
            case "ge":
                exp = Expression.GreaterThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                break;
            case "lt":
                exp = Expression.LessThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                break;
            case "le":
                exp = Expression.LessThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                break;
            case "eq":
                exp = Expression.Equal(propertyAccess, Expression.Constant(searchString.ToType(propertyAccess.Type), propertyAccess.Type));
                break;
            case "ne":
                exp = Expression.NotEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                break;
            default:
                return null;
        }

        return (Expression<Func<T, bool>>)Expression.Lambda(exp, p);
    }

I get the following exception.

LINQ to Entities does not recognize the method ‘System.String ToString(System.String)’ method, and this method cannot be translated into a store expression.

Answer 1

I suspect you want something like this (fixed from previous version):

using System;
using System.Linq.Expressions;

public class Person
{
    public DateTime DateOfBirth { get; set; }
}

public class Test
{
    static void Main()
    {
        var expr = Foo<Person>("DateOfBirth", "1976");

        Person p = new Person
        {
            DateOfBirth = new DateTime(1976, 6, 19)
        };

        Console.WriteLine(expr.Compile()(p));
    }

    static Expression<Func<T, bool>> Foo<T>(string propertyName,
                                            string searchValue)
    {
        ParameterExpression parameter = Expression.Parameter(typeof(T), "x");
        Expression property = Expression.Property(parameter, propertyName);
        Expression toStringCall = Expression.Call(
            property, "ToString",
            null,
            new[] { Expression.Constant("D") });

        Expression containsCall = Expression.Call(
            toStringCall, "Contains",
            null,
            new[] { Expression.Constant(searchValue) });

        return Expression.Lambda<Func<T, bool>>(containsCall, parameter);
    }
}

Note that the “null” values are to show that it’s a non-generic method call.