Is it possible to use SqlGeography with Linq to Sql?

Question

I've been having quite a few problems trying to use Microsoft.SqlServer.Types.SqlGeography. I know full well that support for this in Ling to Sql is not great. I've tried numerous ways, beginning with what would the expected way (Database type of geography, CLR type of SqlGeography). This produces the NotSupportedException, which is widely discussed via blogs.

I've then gone down the path of treating the geography column as a varbinary(max), as geography is a UDT stored as binary. This seems to work fine (with some binary reading and writing extension methods).

However, I'm now running into a rather obscure issue, which does not seem to have happened to many other people.

System.InvalidCastException: Unable to cast object of type 'Microsoft.SqlServer.Types.SqlGeography' to type 'System.Byte[]'.

This error is thrown from an ObjectMaterializer when iterating through a query. It seems to only occur when the tables containing geography columns are included in a query implicitly (ie. using the EntityRef<> properties to do joins).

System.Data.Linq.SqlClient.ObjectReaderCompiler.ObjectReader`2.MoveNext()

My question: If I'm retrieving the geography column as varbinary(max), I might expect the reverse error: can't cast byte[] to SqlGeography. That I would understand. This I don't. I do have some properies on the partial LINQ to SQL classes that hide the binary conversion... could those be the issue?

Any help appreciated, and I know there's probably not enough information.

Extras:

• A geography column in the Visual Studio dbml Designer with 'Server Data Type' = geography generates this error: The specified type 'geography' is not a valid provider type.
• A geography column in the Visual Studio dbml Designer with no 'Server Data Type' generates this error: Could not format node 'Value' for execution as SQL.

Answer 1

Spatial types are not supported by Linq to SQL. Support is not "not great" - it's nonexistent.

You can read them as BLOBs, but you can't do that by simply changing the column type in Linq to SQL. You need to alter your queries at the database level to return the column as a varbinary, using the CAST statement. You can do this at the table level by adding a computed varbinary column, which Linq will happily map to a byte[].

In other words, some DDL like this:

ALTER TABLE FooTable
ADD LocationData AS CAST(Location AS varbinary(max))

Then, remove the Location column from your Linq to SQL class, and use LocationData instead.

If you then need access to the actual SqlGeography instance, you'll need to convert it to and from the byte array, using STGeomFromWKB and STAsBinary.

You can make this process a bit more "automatic" by extending the partial Linq to SQL entity class and adding an auto-converting property:

public partial class Foo
{
    public SqlGeography Location
    {
        get { return SqlGeography.STGeomFromWKB(LocationData, 4326); }
        set { LocationData = value.STAsBinary(); }
    }
}

This assumes that LocationData is the name of the computed varbinary column; you don't include the "real" Location column in your Linq to SQL definition, you add it in the ad-hoc fashion above.

Note also that you won't be able to do much with this column other than read and write to it; if you try to actually query on it (i.e. including it in a Where predicate) then you'll just get a similar NotSupportedException.

Answer 2

If all you want to do with SqlGeography is track points and take advantage of SQL Server 2008's spatial indices, you can, as others have noted, hide your spatial data column from Linq to SQL and use UDFs or stored procedures. Suppose that you have a table AddressFields that includes Latitude and Longitude fields. Add that table to your DBML file, and write any code that you want that sets the Latitude and Longitude fields. Then, the SQL code below will add a Geo geogarphy field to that table and create a trigger in the database that automatically sets the Geo field based on the Latitude and Longitude fields. Meanwhile, the code below also creates other useful UDFs and stored procedures: DistanceBetween2 (I already had a DistanceBetween) returns the distance between the address represented in an AddressField and a specified latitude/longitude pair; DistanceWithin returns various fields from all AddressFields within a specified mile distance; UDFDistanceWithin does the same as a user-defined function (useful if you want to embed this in a larger query); and UDFNearestNeighbors returns fields from AddressField corresponding to the specified number of neighbors nearest a particular point. (One reason for using UDFNearestNeighbors is that SQL Server 2008 won't optimize its use of a spatial index if you just call order by calling DistanceBetween2.)

You'll need to customize this by changing AddressFields to your table and customizing the fields from that table that you want returned (look in the code around references to AddressFieldID). You can then run this on your database and copy the resulting stored procedures and UDFs onto your DBML, and then you can use them in queries. Overall, this allows you to take advantage of a spatial index of points fairly easily.

-----------------------------------------------------------------------------------------

--[1]

--INITIAL AUDIT
select * from dbo.AddressFields
GO
--ADD COLUMN GEO
IF EXISTS (SELECT name FROM sysindexes WHERE name = 'SIndx_AddressFields_geo')
DROP INDEX SIndx_AddressFields_geo ON AddressFields
GO
IF EXISTS (SELECT b.name FROM sysobjects a, syscolumns b 
            WHERE a.id = b.id and a.name = 'AddressFields' and b.name ='Geo' and a.type ='U' )  
ALTER TABLE AddressFields DROP COLUMN Geo

GO
alter table AddressFields add Geo geography

---

--[2]

--SET GEO VALUE
GO
UPDATE AddressFields
SET Geo = geography::STPointFromText('POINT(' + CAST([Longitude] AS VARCHAR(20)) + ' ' + 
                    CAST([Latitude] AS VARCHAR(20)) + ')', 4326)

---

--[3] CREATE INDEX

IF EXISTS (SELECT name FROM sysindexes WHERE name = 'SIndx_AddressFields_geo')
DROP INDEX SIndx_AddressFields_geo ON AddressFields

GO

CREATE SPATIAL INDEX SIndx_AddressFields_geo 
   ON AddressFields(geo)

--UPDATE STATS
UPDATE STATISTICS AddressFields

--AUDIT
GO
select * from dbo.AddressFields

---

--[4] CREATE PROCEDURE USP_SET_GEO_VALUE PARA 1 LATITUDE 2 LONGITUDE

IF EXISTS (SELECT name FROM sysobjects  WHERE name = 'USPSetGEOValue' AND type = 'P')
    DROP PROC USPSetGEOValue
GO

GO
CREATE PROC USPSetGEOValue @latitude decimal(18,8), @longitude decimal(18,8)
AS
    UPDATE AddressFields
    SET Geo = geography::STPointFromText('POINT(' + CAST(@longitude AS VARCHAR(20)) + ' ' + 
                    CAST(@latitude AS VARCHAR(20)) + ')', 4326)
    WHERE [Longitude] =@longitude and [Latitude] = @latitude

GO
--TEST
EXEC USPSetGEOValue 38.87350500,-76.97627500

GO

---

--[5] CREATE TRIGGER ON LAT/LONG VALUE CHANGE/INSERT ---> SET GEOCODE

IF EXISTS (SELECT name FROM sysobjects  WHERE name = 'TRGSetGEOCode' AND type = 'TR')
DROP TRIGGER TRGSetGEOCode

GO

CREATE TRIGGER TRGSetGEOCode 
ON AddressFields
AFTER INSERT,UPDATE
AS
    DECLARE @latitude decimal(18,8), @longitude decimal(18,8)

    IF ( UPDATE (Latitude) OR UPDATE (Longitude) )
        BEGIN

            SELECT @latitude = latitude ,@longitude = longitude from inserted

            UPDATE AddressFields
            SET Geo = geography::STPointFromText('POINT(' + CAST(@longitude AS VARCHAR(20)) + ' ' + 
                        CAST(@latitude AS VARCHAR(20)) + ')', 4326)
            WHERE [Longitude] =@longitude and [Latitude] = @latitude
        END 
    ELSE
        BEGIN
            SELECT @latitude = latitude ,@longitude = longitude from inserted

            UPDATE AddressFields
            SET Geo = geography::STPointFromText('POINT(' + CAST(@longitude AS VARCHAR(20)) + ' ' + 
                        CAST(@latitude AS VARCHAR(20)) + ')', 4326)
            WHERE [Longitude] =@longitude and [Latitude] = @latitude
        END 
GO

---

--[6] CREATE PROC USP_SET_GEO_VALUE_INITIAL_LOAD ----> ONE TIME RUN ONLY

IF EXISTS (SELECT name FROM sysobjects  WHERE name = 'USPSetAllGeo' AND type = 'P')
    DROP PROC USPSetAllGeo
GO

CREATE PROC USPSetAllGeo
AS
UPDATE AddressFields
SET Geo = geography::STPointFromText('POINT(' + CAST([Longitude] AS VARCHAR(20)) + ' ' + 
                    CAST([Latitude] AS VARCHAR(20)) + ')', 4326)

GO

---

--[7] EXISTING PROC DistanceBetween, which returns the distance between two points specified

--by latitude/longitude coordinate pairs. --ALTER PROC DistanceBetween2

IF EXISTS (SELECT name FROM sysobjects  WHERE name = 'DistanceBetween2' AND type = 'FN')
DROP FUNCTION DistanceBetween2

GO

CREATE FUNCTION [dbo].[DistanceBetween2] 
(@AddressFieldID as int, @Lat1 as real,@Long1 as real)
RETURNS real
AS
BEGIN

    DECLARE @KMperNM float = 1.0/1.852;

    DECLARE @nwi geography =(select geo from addressfields where AddressFieldID  = @AddressFieldID)

    DECLARE @edi geography = geography::STPointFromText('POINT(' + CAST(@Long1 AS VARCHAR(20)) + ' ' + 
                                CAST(@Lat1 AS VARCHAR(20)) + ')', 4326)

    DECLARE @dDistance as real = (SELECT (@nwi.STDistance(@edi)/1000.0) * @KMperNM)

    return (@dDistance);  

END

GO --TEST

DistanceBetween2 12159,40.75889600,-73.99228900

---

--[8] CREATE PROCEDURE USPDistanceWithin

-- RETURNS LIST OF ADDRESSES FROM AddressFields table

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'USPDistanceWithin' AND type = 'P') DROP PROCEDURE USPDistanceWithin

GO

CREATE PROCEDURE [dbo].USPDistanceWithin 
(@lat as real,@long as real, @distance as float)
AS
BEGIN

    DECLARE @edi geography = geography::STPointFromText('POINT(' + CAST(@Long AS VARCHAR(20)) + ' ' + 
                                CAST(@Lat AS VARCHAR(20)) + ')', 4326)

    SET @distance = @distance * 1609.344 -- convert distance into meter

    select 
         AddressFieldID
        ,FieldID
        ,AddressString
        ,Latitude
        ,Longitude
        ,LastGeocode
        ,Status
        --,Geo
    from 
        AddressFields a WITH(INDEX(SIndx_AddressFields_geo))
    where 
        a.geo.STDistance(@edi) < = @Distance 

END

GO

--TEST

--within 3 miles USPDistanceWithin 38.90606200,-76.92943500,3 GO --within 5 miles USPDistanceWithin 38.90606200,-76.92943500,5 GO --within 10 mile USPDistanceWithin 38.90606200,-76.92943500,10

---

--[9] CREATE FUNCTION FNDistanceWithin

-- RETURNS LIST OF ADDRESSES FROM AddressFields table

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'UDFDistanceWithin' AND type = 'TF') DROP FUNCTION UDFDistanceWithin

GO

CREATE FUNCTION UDFDistanceWithin 
(@lat as real,@long as real, @distance as real)
RETURNS @AddressIdsToReturn TABLE 
    (
         AddressFieldID INT
        ,FieldID INT
    )
AS
BEGIN

    DECLARE @edi geography = geography::STPointFromText('POINT(' + CAST(@Long AS VARCHAR(20)) + ' ' + 
                                CAST(@Lat AS VARCHAR(20)) + ')', 4326)

    SET @distance = @distance * 1609.344 -- convert distance into meter

    INSERT INTO @AddressIdsToReturn
    select 
         AddressFieldID
        ,FieldID
    from 
        AddressFields a WITH(INDEX(SIndx_AddressFields_geo))
    where 
        a.geo.STDistance(@edi) < = @Distance 

    RETURN 

END

GO

--TEST

--within 3 miles select * from UDFDistanceWithin(38.90606200,-76.92943500,3) GO --within 5 miles select * from UDFDistanceWithin( 38.90606200,-76.92943500,5) GO --within 10 mile select * from UDFDistanceWithin( 38.90606200,-76.92943500,10)

---

--[9] CREATE FUNCTION UDFNearestNeighbors

-- RETURNS LIST OF ADDRESSES FROM AddressFields table

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'UDFNearestNeighbors' AND type = 'TF') DROP FUNCTION UDFNearestNeighbors

GO

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'numbers' AND xtype = 'u') DROP TABLE numbers

GO
-- First, create a Numbers table that we will use below.
SELECT TOP 100000 IDENTITY(int,1,1) AS n INTO numbers FROM MASTER..spt_values a, MASTER..spt_values b CREATE UNIQUE CLUSTERED INDEX idx_1 ON numbers(n)

GO

CREATE FUNCTION UDFNearestNeighbors 
(@lat as real,@long as real, @neighbors as int)
RETURNS @AddressIdsToReturn TABLE 
    (
         AddressFieldID INT
        ,FieldID INT
    )
AS
BEGIN

    DECLARE @edi geography = geography::STPointFromText('POINT(' + CAST(@Long AS VARCHAR(20)) + ' ' + 
                                CAST(@Lat AS VARCHAR(20)) + ')', 4326)
    DECLARE @start FLOAT = 1000;

    WITH NearestPoints AS

    (

      SELECT TOP(@neighbors) WITH TIES *,  AddressFields.geo.STDistance(@edi) AS dist

      FROM Numbers JOIN AddressFields WITH(INDEX(SIndx_AddressFields_geo)) 

      ON AddressFields.geo.STDistance(@edi) < @start*POWER(2,Numbers.n)

      ORDER BY n

    )


    INSERT INTO @AddressIdsToReturn

    SELECT TOP(@neighbors)
         AddressFieldID
        ,FieldID
    FROM NearestPoints
    ORDER BY n DESC, dist

    RETURN 

END

GO

--TEST

--50 neighbors select * from UDFNearestNeighbors(38.90606200,-76.92943500,50) GO --200 neighbors select * from UDFNearestNeighbors( 38.90606200,-76.92943500,200) GO