如何使用 Dapper 从数据库 map 到字典？

Question

I have a class:

public class TestClass
{
    public string value1 {get; set;}
    public string value2 {get; set;}
    public string value3 {get; set;}
    public string value4 {get; set;}
}

and the database:

Database - Test

id    code    value1    value2    value3     value4
--    ----    ------    ------    ------     ------

1     1000    xxxxx     xxxxx     xxxxx      xxxxx
2     1000    yyyyy     .....     .....      .....
3     1000    yyyy3     .....     .....      .....
4     1000    yyyy4
5     2000    yyyy5
6     2000    yyyy6
7     3000    yyyy7
8     3000    yyyy8
9     4000    yyyy9
10    4000    y9999

This is gonna be 4 key and 4 list of TestClass. string is a code and rest is testClass list.

I wanna map this to this dictionary: If Code is a same add it into list of TestClass.

Dictionary<string, List<TestClass>> testDic = new Dictionary<string, List<TestClass>>();

testDic = conn.Query("SELECT * FROM Test").ToDictionary(x=> x.????) ;

How to do it? I assume it is something like think but its not working?

Answer 1

It looks like you're trying to group by the code . To do that, I would do something like:

public class TestClass
{
    public int Id {get;set;}
    public string Code {get;set;}
    public string Value1 {get; set;}
    public string Value2 {get; set;}
    public string Value3 {get; set;}
    public string Value4 {get; set;}
}
/// ...
testDic = conn.Query<TestClass>("SELECT * FROM Test").ToLookup(x=> x.Code);

The ToLookup here is standard LINQ which works a lot like a Dictionary<TKey, List<TValue>> , but inbuilt (it is essentially a lot like GroupBy , but intended to be accessed multiple times).

If you actually need a dictionary, it is more complex; ToDictionary won't really help you, so the most practical approach is probably:

var data = new Dictionary<string, List<TestClass>>();
foreach (var row in conn.Query<TestClass>("SELECT * FROM Test"))
{
    if (!data.TryGetValue(row.Code, out var list))
    {
        data.Add(row.Code, list = new List<TestClass>());
    }
    list.Add(row);
}