如何使用Select从LINQ查询中获取数据

Question

I'm testing a simple LINQ select query and want to get two DateTime values from the table, but I'm doing something wrong here and need to know what I'm doing/thinking wrong?

My query:

var test = from x in db.Projects where x.ID == 1 select x;

Then I try to get on of the values like this:

DateTime Date = test. ????? 

Here I thought I should get a suggestion from the Intellisense after the dot to pick the value from the column StartDate the table, but this isn't working.

Answer 1

If you need multiple matches...

Are you sure that you have multiple Project objects that have the same ID of 1 which your query currently suggests? If that is the case, then your query should return all of the records that meet that constraint via the Where() method :

// Get your Projects that meet your criteria
var test = db.Projects.Where(p => p.ID == 1);

If you need to access properties from these elements, you could either loop through them explicitly :

// Iterate through each match that was found
foreach(var t in test)
{
      // Access your properties here
      DateTime dt = t.YourDateProperty;
}

Or you could accomplish this using a Select() statement to only pull the properties that you need :

// This will return a collection of Dates mapped from each element in your collection
var testDates = db.Projects.Where(p => p.ID == 1)
                           .Select(x => x.YourDateProperty);

If you only need a single match...

If you only need to match a single element within your collection, you might consider using the First() , Single() or their equivalent FirstOrDefault() and SingleOrDefault() methods, which will return a single entity that you can use as expected :

// This will return the first Project with an ID of 1
var test = db.Project.FirstOrDefault(p => p.ID == 1);
// If it was successful
if(test != null)
{
     // Then you can safely access it here
     DateTime dt = test.YourDateProperty;
}

The only difference between the methods mentioned (normal vs OrDefault() ) is that the OrDefault() methods will return null if no matching elements are found, so they generally require a null check as seen above.

Answer 2

test is going to be an enumeration ( IEnumerable<> , IQueryable<> , etc... many are applicable) of your Project type. So if, for example, you want the first record, you might do this:

DateTime Date = test.First().SomeDateProperty;

All of the data returned from your query is in test . It could be zero records, one record, many records, etc.

Answer 3

this is such as:

 var date = test.FirstOrDefault();
 DateTime? Date = date != null ? date.StartDate : null;

Answer 4

In test you will have a collection which matches the condition x.ID == 1 . You should iterate through that collection and take your needed properties.

---

Edit

I suggest you to use the syntax: var result = db.Projects.FirstOrDefault(x => x.ID ==1);