Dapper official translation Tutorial 4: QueryFirst Dapper method of (turn)

Dapper official translation Tutorial 4: QueryFirst Dapper method of

February 28, 2019 10:21:40  Day_and_Night_2017  Reads: 174

 

QueryFirst method described

 

QueryFirst method is an extension method, it can be called IDbConnection object, you can execute a query, and mapped to the result.

You can be mapped type:

• Anonymous (anonymous type var)
• Strongly Typed (strongly typed)

 

The method may be used parameter QueryFirst

 

QueryFirst method Parameter Description

parameter name	Parameter Description
sql	Database statement
param	Query parameters
transaction	Services used
commandTimeout	Execution timeout
commandType	Statement Types

 

First，FirstOrDefault，Single，SingleOrDefault对比

 

Result	No Item	One Item	Many Items
First	Exception	Item	First Item
Single	Exception	Item	Exception
FirstOrDefault	Default	Item	First Item
SingleOrDefault	Default	Item	Exception

The difference between xx and xxOrDefault: xx at the time could not find the data throws an exception, but xxOrDefault will return to the default value, so when there is no uncertainty values, priority use xxOrDefault. 

When containing a plurality of values, and Single SingleOrDefault will raise an exception, or FirstOrDefault First and returns a value that satisfies the first condition.

In summary, priority use FirstOrDefault

 

Anonymous queries

 

string sql = "SELECT * FROM OrderDetails WHERE OrderDetailID = @OrderDetailID;";

using (var connection = new SqlCeConnection("Data Source=SqlCe_W3Schools.sdf"))
{
var orderDetail = connection.QueryFirst(sql, new {OrderDetailID = 1});

FiddleHelper.WriteTable(orderDetail);
}

 

Strongly typed queries

 

string sql = "SELECT * FROM OrderDetails WHERE OrderDetailID = @OrderDetailID;";

using (var connection = new SqlCeConnection("Data Source=SqlCe_W3Schools.sdf"))
{
var orderDetail = connection.QueryFirst(sql, new {OrderDetailID = 1});

FiddleHelper.WriteTable(orderDetail);
}