Searching a DateTime using a search string in Entity Framework code first

Question

I have an MVC project which outputs a nullable DateTime (DateTime?) as a string to the user for each model item as shown below (my user interface has paging so I only do this computation for a small number of records - in other words, this part is okay):

foreach (DocumentEvent item in Model.items)
    @(item?.TimeUtc?.ToString() ?? "N/A")

I want to add a search functionality. I've tried searching as follows, but this is not performant because AsEnumerable materializes my list and I am now in the C# world, enumerating through each record:

using (var context = new ClientEventsContext())
    var items = context.Events.AsEnumerable().Where(x => {
        (x?.TimeUtc?.ToString() ?? "N/A").Contains(model.search)
    });

Instead I want to take advantage of my SQL Server database. How can I build an SQL Server-friendly query for the above code without AsEnumerable that will produce the same results as my current logic?

Answer 1

Here is how you can build and use LINQ to Entities compatible conversion from date to string in M/d/yyyy h:mm:ss tt format. Rather that embedding that monster inside the query, I will use a custom "marker" method and will bind the implementation using ExpressionVisitor. This way you can experiment and change the format if needed (even add some controlling arguments) w/o affecting the readability of the query.

First, the implementation:

public static class EFExtensions
{
    public static string ToCustomDateFormat(this DateTime value)
    {
        // Should never happen
        throw new InvalidOperationException();
    }

    public static IQueryable<T> ApplyCustomDateFormat<T>(this IQueryable<T> source)
    {
        var expression = new CustomDateFormatBinder().Visit(source.Expression);
        if (source.Expression == expression) return source;
        return source.Provider.CreateQuery<T>(expression);
    }

    class CustomDateFormatBinder : ExpressionVisitor
    {
        protected override Expression VisitMethodCall(MethodCallExpression node)
        {
            if (node.Method.DeclaringType == typeof(EFExtensions) && node.Method.Name == "ToCustomDateFormat")
            {
                var date = Visit(node.Arguments[0]);
                var year = DatePart(date, v => DbFunctions.Right("0000" + v.Year, 4));
                var month = DatePart(date, v => v.Month.ToString());
                var day = DatePart(date, v => v.Day.ToString());
                var hour = DatePart(date, v => (1 + (v.Hour + 11) % 12).ToString());
                var minute = DatePart(date, v => DbFunctions.Right("0" + v.Minute, 2));
                var second = DatePart(date, v => DbFunctions.Right("0" + v.Second, 2));
                var amPM = DatePart(date, v => v.Hour < 12 ? "AM" : "PM");
                var dateSeparator = Expression.Constant("/");
                var timeSeparator = Expression.Constant(":");
                var space = Expression.Constant(" ");
                var result = Expression.Call(
                    typeof(string).GetMethod("Concat", new Type[] { typeof(string[]) }),
                    Expression.NewArrayInit(typeof(string),
                        month, dateSeparator, day, dateSeparator, year, space,
                        hour, timeSeparator, minute, timeSeparator, second, space, amPM));
                return result;    
            }
            return base.VisitMethodCall(node);
        }

        Expression DatePart(Expression date, Expression<Func<DateTime, string>> part)
        {
            var parameter = part.Parameters[0];
            parameterMap.Add(parameter, date);
            var body = Visit(part.Body);
            parameterMap.Remove(parameter);
            return body;
        }

        Dictionary<ParameterExpression, Expression> parameterMap = new Dictionary<ParameterExpression, Expression>();

        protected override Expression VisitParameter(ParameterExpression node)
        {
            Expression replacement;
            return parameterMap.TryGetValue(node, out replacement) ? replacement : node;
        }
    }
}

then the usage:

var items = context.Events
    .Where(x => x.TimeUtc != null && 
        x.TimeUtc.Value.ToCustomDateFormat().Contains(model.search))
    .ApplyCustomDateFormat();

Answer 2

I found a solution. This syntax seems to work:

using (var context = new ClientEventsContext())
    var items = context.Events.Where(x => {
        x.TimeUtc.HasValue && x.TimeUtc.Value.ToString().Contains(model.search)
    });