How can I call a SQL function in LINQ to SQL using LINQPad?

Thanks Dluk.
This is a little advanced for me. I wasn't sure where to I put this code.
So I entered it at the top of my script, and now my script looks like this:

[LinqToDB.Sql.Function(Name="member_type_on_date", ServerSideOnly=true)]
public static string member_type_on_date(int? id, string date) => throw new InvalidOperationException();

var query1 = (from c in customers
where c.id == 112
select new
{
c.id,
member_type_on_date(id, "2020-03-31")
}).ToArray();
query1.Dump("ID and Member Type");

But I don't understand why it shwos an error, underlining the "var" keyword at "var query1 ... " and the errors is:
"CS0825 The contextual keywoard 'var' may only appear within a local variable declaration or in script code"

Thank you,
Steven

Thanks Dluk.

Am I adding that to the end of that file?:
void Main()
{
// Write code to test your extensions here. Press F5 to compile and run.
}
public static class MyExtensions
{
// Write custom extension methods here. They will be available to all queries.
}
// You can also define non-static classes, enums, etc.
[LinqToDB.Sql.Function(Name = "member_type_on_date", ServerSideOnly = true)]
public static string member_type_on_date(int? id, string date) => throw new InvalidOperationException();

it shows me some errors now there:
CS0246 The type or namespace name 'LinqToDB' could not be found (press F4 to add a using directive or assembly reference)
CS0246 The type or namespace name 'Name' could not be found (press F4 to add a using directive or assembly reference)
CS0246 The type or namespace name 'ServerSideOnly' could not be found (press F4 to add a using directive or assembly reference)

Do I need to add reference to some .dll files?

Thanks Dluk.

I saw that the driver file "linq2db.LINQPad.lpx" is actually a zip file and found the "linq2db.dll" file inside.
Copied to Program Files\LINQPad folder and added a reference to that.
Now I don't have those 3 errors anymore, and I only have 1 error now when I try to run the script:
"Cannot execute text selection: CS0103 The name 'member_type_on_date' does not exist in the current context"
and highlighting line number 5:

var query1 = (from c in customers
where c.id == 112
select new {
c.id,
member_type_on_date = member_type_on_date(id, "2020-03-31")
}).ToArray();
query1.Dump("ID and Member Type");

Thanks a lot for your help,
Steven

Hi Dluk,

Sorry I still cannot make it working, I'm a bit new to C#.
So you mean like this?
var query1 = (from c in customers
where c.id == 112
select new {
c.id,
member_type_on_date = MyExtensions.member_type_on_date(id, "2020-03-31")
}).ToArray();
query1.Dump("ID and Member Type");

I am till getting an error:
"Cannot execute text selection: CS0117 'MyExtensions' does not contain a definition for 'member_type_on_date' "

But I have this already in MyExtensions file and ran it and it compiles:
void Main()
{
// Write code to test your extensions here. Press F5 to compile and run.
}
public static class MyExtensions
{
// Write custom extension methods here. They will be available to all queries.
}
// You can also define non-static classes, enums, etc.
[LinqToDB.Sql.Function(Name = "member_type_on_date", ServerSideOnly = true)]
public static string member_type_on_date(int? id, string date) => throw new InvalidOperationException();

Thanks,
Steven

OK, thanks, I moved it to MyExtensions class now.

Now I am getting a different kind of error:
"InvalidOperationException: Operation is not valid due to the current state of the object."
It's in this line:
"var query1 = (from c in customers"

Here is the StackTrace:
at MyExtensions.member_type_on_date(Nullable1 id, String date) at lambda_method(Closure , FieldReader ) at IQToolkit.Data.DbEntityProvider.Executor.<Project>d__101.MoveNext()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) at IQToolkit.Data.DbEntityProvider.Executor.Execute[T](QueryCommand command, Func2 fnProjector, MappingEntity entity, Object[] paramValues)
at IQToolkit.Data.EntitySession.SessionExecutor.Execute[T](QueryCommand command, Func2 fnProjector, MappingEntity entity, Object[] paramValues) at lambda_method(Closure ) at IQToolkit.Data.EntityProvider.Execute(Expression expression) at IQToolkit.Data.EntitySession.SessionProvider.Execute(Expression expression) at IQToolkit.Query1.GetEnumerator()
at System.Linq.Buffer1..ctor(IEnumerable1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at UserQuery.Main() in C:\Users\snagy\AppData\Local\Temp\LINQPad5_uwcpnzdn\query_ljblun.cs:line 41
at LINQPad.ExecutionModel.ClrQueryRunner.Run()
at LINQPad.ExecutionModel.Server.RunQuery(QueryRunner runner)
at LINQPad.ExecutionModel.Server.StartQuery(QueryRunner runner)
at LINQPad.ExecutionModel.Server.<>c__DisplayClass153_0.b__0()
at LINQPad.ExecutionModel.Server.SingleThreadExecuter.Work()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

The Source shows: MyExtensions.FW46
and HResult shows: -2146233079

Thanks,
Steven

Thanks Dluk, I didn't notice that.
It works everything now.
Thanks a lot for your help.
Steven