How to execute a function parameter in C#

Question

I am trying to create a generic method that will take a Func parameter.

traditional func is that we create methodOne() and send it as a parameter to another method. However, that will defeat the purpose of making the method I am trying to create as a generic.

My code:

public static List<T> ExecuteQuery<T>(string connection, 
                                      string commandText, 
                                      Func<SqlDataReader, List<T>> myMethodName)
{
    List<T> items = new List<T>();
    SqlDataReader sqlDataReader = null;

    try
    {
        using (SqlConnection con = new SqlConnection(connection))
        {
            using (SqlCommand cmd = new SqlCommand(commandText, con))
            {
                try
                {
                    con.Open();
                    sqlDataReader = cmd.ExecuteReader();

                    items = myMethodName(sqlDataReader);
                }
                catch (Exception ex)
                {
                    if (sqlDataReader != null) sqlDataReader.Close();

                    cmd.Dispose();

                    throw ex;
                }
                finally
                {
                    if (sqlDataReader != null) sqlDataReader.Dispose();

                    cmd.Dispose();
                }
            }
        }
    }
    catch (Exception)
    {
        throw;
    }

    return items;
}

Trying to call the method above like this:

    public List<Function> GetDeletedFunctions(string connectionString)
    {
        SqlDataReader sqlDataReader = null;
        List<Function> functions;

        string cmdText = @"SELECT * FROM Table "; // dumy query

        functions = DbHelper.ExecuteQuery<Function>(
          connectionString, 
          cmdText, 
          List<Function>(sqlDataReader)
        {

            var f =
            (from x in sqlDataReader.Cast<DbDataRecord>()
             select new Function
             {
                 Param1 = DbHelper.GetValue<string>("Param1 ", x),
                 Param2 = DbHelper.GetValue<string>("Param2", x),
             }).ToList();

            return f;
        } );
}

Compile Time Error:

Error CS1955 Non-invocable member 'List' cannot be used like a method.

I am assuming there is a syntax issue here and its driving me nuts. Any suggestions?

First off you don´t assign anything to your sqlDataReader. — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Mar 15, 2019 at 9:45

Dmitrii Bychenko · Accepted Answer · 2019-03-18 07:14:11Z

2

When wrapping into using you don't have to catch exceptions (.Net will free resources for you, rain on shine)

// Dangerous practice: what if I put commandText "drop table MyTable"?
public static List<T> ExecuteQuery<T>(string connection, 
                                      string commandText, 
                                      Func<SqlDataReader, List<T>> myMethodName) {
  //ToDo: Validate parameters here

  using (var con = new SqlConnection(connection)) {
    con.Open();

    using (var cmd = new SqlCommand(commandText, con)) {
      // IDataReader is IDisposable as well       
      using (var reader = cmd.ExecuteReader()) {
        return myMethodName(reader);
      } 
    }
  }  
}

Then all we have to do is to provide arguments: connectionString, commandText and map function; let's do it:

public List<Function> GetDeletedFunctions(string connectionString) {
  //TODO: Validate connectionString here 

  return DbHelper.ExecuteQuery<Function>(
     connectionString,                                // Connection String
   @"SELECT * FROM Table ",                           // Query
    (reader) => {                                     // Map function
       // When given reader, the map function creates list
       List<Function> result = new List<Function>();

       // Convert each record into Function instance and add them into the list
       while (reader.Read()) {
         Function item = new Function() {
           Param1 = Convert.ToString(reader["Param1"]),
           Param2 = Convert.ToString(reader["Param2"]),
         }

         result.Add(item); 
       }

       // and return the list
       return result;
    }
  )
}

edited Mar 18, 2019 at 7:14

answered Mar 15, 2019 at 10:01

Dmitrii Bychenko

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6 Comments

ThunD3eR Over a year ago

Excellent answer! Thank you, If I could just bother you with one thing: " (reader) => " this syntax is the equivalent of? and also the "reader" does not exist as a parameter anywhere in the code but I am not getting any errors, how come?

Sarvesh Mishra Over a year ago

reader is a parameter being passed to a function written after => (Lambda Expression)

Dmitrii Bychenko Over a year ago

(reader) => {body} is typical syntax of a lambda function where reader is formal parameter (you can change it into, say, r)

ThunD3eR Over a year ago

Again thank you! tend to use lambada expressions when doing the linq queries but never done it as a function like this.

Dmitrii Bychenko Over a year ago

@ThunD3eR: delegate is a typical alternative to lambda function. learn.microsoft.com/en-us/dotnet/api/…

|

Sarvesh Mishra · Accepted Answer · 2019-03-15 10:10:49Z

There is problem in the way you are trying to pass function as parameter. Please use the following code:

public List<Function> GetDeletedFunctions(string connectionString)
{                        
    string cmdText = @"SELECT * FROM Table "; // dumy query
    return DbHelper.ExecuteQuery<Function>(connectionString,
                                           cmdText,
                                           (SqlDataReader sqlDataReader) =>
                                           {
                                               var f = (from x in sqlDataReader.Cast<DbDataRecord>()
                                               select new Function
                                               {
                                                   Param1 = DbHelper.GetValue<string>("Param1 ", x),
                                                   Param2 = DbHelper.GetValue<string>("Param2", x)
                                               }).ToList();

                                               return f;
                                           });
}

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