Execute stored procedure w/parameters in Dapper

var queryParameters = new DynamicParameters();
queryParameters.Add("@parameter1", valueOfparameter1);
queryParameters.Add("@parameter2", valueOfparameter2);

await db.QueryAsync<YourReturnType>(
    "{NameOfStoredProcedure}",
    queryParameters,
    commandType: CommandType.StoredProcedure)

You need to tell it the command type: make sure there's a commandType: CommandType.StoredProcedure in the dapper call. Otherwise, it is simply executing the text command:

spMyStoredProcedure

(with some unused parameters in the ambient context). This is legal TSQL, and attempts to call spMyStoredProcedure without passing parameters - the same as if you put spMyStoredProcedure into SSMS and press f5.

Also, if your parameters are fixed, I would actually suggest just using:

var param = new { somethingId };

or even just inline it completely:

var result = repository.Exec<Something>(SomethingEnum.spMyStoredProcedure,
    new { somethingId }, commandType: CommandType.StoredProcedure);

(note: if your Exec<T> method only ever handles stored procedures, you could move the commandType internal to the method - or you could make it an optional parameter that defaults to CommandType.StoredProcedure)

Since this was the top result for me, but there were no answers that deal with ExecuteNonQuery with table valued parameters, here is the code for that:

var queryParameters = new DynamicParameters();
queryParameters.Add("@Param0", datatable0.AsTableValuedParameter());
queryParameters.Add("@Param1", datatable1.AsTableValuedParameter());
var result = await ExecuteStoredProc("usp_InsertUpdateTest", queryParameters);

private async Task<Result<int>> ExecuteStoredProc(string sqlStatement, DynamicParameters parameters)
    {
        try
        {
            using (SqlConnection conn = new SqlConnection(connectionString))
            {
                await conn.OpenAsync();
                var affectedRows = await conn.ExecuteAsync(
                    sql: sqlStatement,
                    param: parameters,
                    commandType: CommandType.StoredProcedure);
                return Result.Ok(affectedRows);
            }
        }
        catch (Exception e)
        {
            //do logging
            return Result.Fail<int>(e.Message);
        }
    }