The ExecuteReader method of the Dapper library is a versatile method you can use to execute SQL statements and map the results to an instance of type IDataReader. This enables developers to easily read data from databases in applications written using C#, VB.NET, or other .NET languages.
The ExecuteReader method has a variety of overloads that allow developers to specify the command type, parameters, transaction, and command timeout. The following table shows the different parameters of an ExecuteReader method.
| Name | Description |
|---|---|
| sql | It represents an SQL query or stored procedure. This parameter is required. |
| param | It represents the parameters required by SQL query or stored procedure. This parameter is optional. We can pass the parameter to SQL in anonymous type, dynamic object, or Dapper.DynamicParameters class. |
| transaction | It represents a database transaction. This parameter is optional if we use this method outside of a transaction. Otherwise, it is required. |
| commandTimeout | It represents the time in seconds to wait before terminating the command execution and generating an error. The default value of this parameter is 30 seconds. This parameter is optional. |
| commandType | It specifies how SQL query or stored procedure should be interpreted by the data provider. The default value of this parameter is CommandType.Text. This parameter is optional. |
Dapper ExecuteReader
The following example shows how to use the ExecuteReader method. First, we create a connection to the database using a SqlConnection object and then create an SQL query to select the data from the database and pass it to the ExecuteReader method of Dapper.
var sql = "SELECT * FROM Product WHERE CategoryID = @categoryID"; var reader = connection.ExecuteReader(sql, new { categoryID = 1 }); while(reader.Read()) { var productId = reader.GetInt32(reader.GetOrdinal("ProductID")); var name = reader.GetString(reader.GetOrdinal("Name")); Console.WriteLine($"ProductID: {productId}; Name: {name}"); } You can also fill a DataTable or DataSet from the results returned by the ExecuteReader method.
var sql = "SELECT * FROM Product WHERE CategoryID = @categoryID"; var reader = connection.ExecuteReader(sql, new { categoryID = 1 }); DataTable table = new DataTable(); table.Load(reader); Dapper ExecuteReaderAsync
To execute a reader asynchronously, Dapper provides the ExecuteReaderAsync method, an asynchronous version of the ExecuteReader method.
var sql = "SELECT * FROM Product WHERE CategoryID = @categoryID"; var reader = await connection.ExecuteReaderAsync(sql, new { categoryID = 1 }); while(reader.Read()) { var productId = reader.GetInt32(reader.GetOrdinal("ProductID")); var name = reader.GetString(reader.GetOrdinal("Name")); Console.WriteLine($"ProductID: {productId}; Name: {name}"); } 
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