LINQ to Entities does not recognize the method 'System.TimeSpan Subtract(System.DateTime)

Question

The following throws an error:

public FieldViewer GetFieldViewByFieldIDIPAndUserByDate( int fieldID, string ip, string userID, DateTime date )
{
    return this.         


        

Answer 1

Try this to find the difference in minutes:

EntityFunctions.DiffMinutes(date, x.Viewer.CreatedAt) >= 10

Sources:

How to subtract EntityFunctions.TruncateTime from custom time

EntityFunctions.DiffMinutes

As this api is now obsolete (thanks for pointing this out @Jimmyt1988), instead use: DbFunctions.DiffMinutes

Answer 2

EntityFunctions.DiffMinutes for your case will be inefficient. You should do this way

public FieldViewer GetFieldViewByFieldIDIPAndUserByDate( int fieldID, string ip, string userID, DateTime date )
{
    var tenMinThreshold = date - TimeSpan.FromMinutes(10);
    return this.context.FieldViewers.Where( x =>
        x.Field.FieldID == fieldID &&
        x.Viewer.IPAddress == ip &&
        x.Viewer.User.Id == userID &&
        x.Viewer.CreatedAt <= tenMinThreshold).FirstOrDefault();
}

Answer 3

One way to fix this is to perform that part of the filtering outside of the LINQ to Entities provider, using the LINQ to Objects provider. To do that, append a call to AsEnumerable() before that operation:

public FieldViewer GetFieldViewByFieldIDIPAndUserByDate( int fieldID, string ip, string userID, DateTime date )
{
    return this.context.FieldViewers.Where( x =>
            x.Field.FieldID == fieldID &&
            x.Viewer.IPAddress == ip &&
            x.Viewer.User.Id == userID)
       .AsEnumerable()
       .Where(x => date.Subtract( x.Viewer.CreatedAt ).TotalMinutes >= 10)
       .FirstOrDefault();  
}

Another way is to use one of the specialized LINQ to Entities operations, like DiffMinutes.