How do I perform an insert and return inserted identity with Dapper?

Question

It does support input/output parameters (including RETURN value) if you use DynamicParameters, but in this case the simpler option is simply:

var id = connection.QuerySingle<int>( @"
INSERT INTO [MyTable] ([Stuff]) VALUES (@Stuff);
SELECT CAST(SCOPE_IDENTITY() as int)", new { Stuff = mystuff});

Note that on more recent versions of SQL Server (2005+) you can use the OUTPUT clause:

var id = connection.QuerySingle<int>( @"
INSERT INTO [MyTable] ([Stuff])
OUTPUT INSERTED.Id
VALUES (@Stuff);", new { Stuff = mystuff});

KB:2019779,"You may receive incorrect values when using SCOPE_IDENTITY() and @@IDENTITY", The OUTPUT clause is the safest mechanism:

string sql = @"
DECLARE @InsertedRows AS TABLE (Id int);
INSERT INTO [MyTable] ([Stuff]) OUTPUT Inserted.Id INTO @InsertedRows
VALUES (@Stuff);
SELECT Id FROM @InsertedRows";

var id = connection.Query<int>(sql, new { Stuff = mystuff}).Single();

A late answer, but here is an alternative to the SCOPE_IDENTITY() answers that we ended up using: OUTPUT INSERTED

Return only ID of inserted object:

It allows you to get all or some attributes of the inserted row:

string insertUserSql = @"INSERT INTO dbo.[User](Username, Phone, Email)
                        OUTPUT INSERTED.[Id]
                        VALUES(@Username, @Phone, @Email);";

int newUserId = conn.QuerySingle<int>(
                                insertUserSql,
                                new
                                {
                                    Username = "lorem ipsum",
                                    Phone = "555-123",
                                    Email = "lorem ipsum"
                                },
                                tran);

Return inserted object with ID:

If you wanted you could get Phone and Email or even the whole inserted row:

string insertUserSql = @"INSERT INTO dbo.[User](Username, Phone, Email)
                        OUTPUT INSERTED.*
                        VALUES(@Username, @Phone, @Email);";

User newUser = conn.QuerySingle<User>(
                                insertUserSql,
                                new
                                {
                                    Username = "lorem ipsum",
                                    Phone = "555-123",
                                    Email = "lorem ipsum"
                                },
                                tran);

---

Also, with this you can return data of deleted or updated rows. Just be careful if you are using triggers because (from link mentioned before):

Columns returned from OUTPUT reflect the data as it is after the INSERT, UPDATE, or DELETE statement has completed but before triggers are executed.

For INSTEAD OF triggers, the returned results are generated as if the INSERT, UPDATE, or DELETE had actually occurred, even if no modifications take place as the result of the trigger operation. If a statement that includes an OUTPUT clause is used inside the body of a trigger, table aliases must be used to reference the trigger inserted and deleted tables to avoid duplicating column references with the INSERTED and DELETED tables associated with OUTPUT.

More on it in the docs: link

The InvalidCastException you are getting is due to SCOPE_IDENTITY being a Decimal(38,0).

You can return it as an int by casting it as follows:

string sql = @"
INSERT INTO [MyTable] ([Stuff]) VALUES (@Stuff);
SELECT CAST(SCOPE_IDENTITY() AS INT)";

int id = connection.Query<int>(sql, new { Stuff = mystuff}).Single();