Dynamic LINQ DateTime Comparison String Building - Linq To Entities

问题

I'm using the dynamic LINQ library by Scott Guthrie together with Entity Framework and C#.

I have to build my where string into a variable based on several factors and then pass the string variable to the where clause. For some reason, this will work:

ContactList = ContactList.Where("DateAdded >= @0", DateTime.Parse("12/1/2012"));

But this will not work

string WhereClause = string.Format("DateAdded >= {0}", DateTime.Parse("12/1/2012"));
ContactList = ContactList.Where(WhereClause);

As mentioned, I need to use it in the version of passing the variable. Anyone know why the second doesn't work?

Thanks in advance!

回答1:

I was able to get it working with a slightly different string format using the information here.

Doing this worked fine for me:

ContactList.Where("DateAdded >= DateTime(2013, 06, 18)")

Note this does not work at all with DateTimeOffset columns.

回答2:

It seems what I was trying to do is not possible with the current DynamicLINQ library. The reason it didn't work was well outlined below by Tilak.

My solution was to modify the DynamicLINQ library to allow the query to be written as a string and passed to the where clause for Date/Time datatypes. The modification was found here by Paul Hatcher: LINQ TO SQL, Dynamic query with DATE type fields

回答3:

ObjectQuery.Where overload accepts 2 parameters.

string predicate
params ObjectParameter[] parameters

In your first example, Where builds the query (where clause) using ObjectParameter parameters (using Name, Type and Value of ObjectParameter)

In your second example, whatever is passed is treated as final where clause (no internal conversion based on datatype of passed parameters done).

回答4:

Based on Richard Rout's option, with a slight modification:

ContactList.Where("DateAdded >= DateTime(2013, 06, 18)")

This works in my Linq2Entities solution. Note the DateTime instead of Date. Hope this saves someone the headache this problem gave me.

回答5:

ContactList.Where probably puts quotes around non-numeric arguments (such as a DateTime). In your second string the date isn't quoted.

来源：https://stackoverflow.com/questions/14126271/dynamic-linq-datetime-comparison-string-building-linq-to-entities

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string

linq

entity-framework

dynamic-linq