在请求1时，QueryAsync()返回什么?单个字符串列的N值?

With this code (with or without the ".ToList()"):

使用此代码(带有或没有“.ToList()”):

public async Task<List<String>> SelectDistinctGroupNames()
{
    var db = new SQLiteAsyncConnection(SQLitePath);
    var groupNames = await db.QueryAsync<List<string>>("SELECT DISTINCT GroupName FROM SOs_Locations");
    return groupNames.ToList();
}

...I get, "Cannot implicitly convert type 'System.Collections.Generic.List>' to 'System.Collections.Generic.List'"

…我得到，“不能隐式地转换类型的系统。集合。通用。”列表>”到“System.Collections.Generic.List’”

This works:

如此:

public async Task<HashSet<String>> SelectDistinctGroupNames()
{
    var db = new SQLiteAsyncConnection(SQLitePath);
    var allLocations = await db.QueryAsync<SOs_Locations>("SELECT * FROM SOs_Locations ORDER BY GroupName");
    HashSet<string> hashsetGroupNames = null;
    foreach (var item in allLocations)
    {
        hashsetGroupNames.Add(item.GroupName);
    }
    return hashsetGroupNames;
}

...but seems wasteful (grabbing all the records, when all I want is distinct values from the GroupName column).

…但是看起来很浪费(当我想要从GroupName列中得到不同的值时，获取所有的记录)。

It seems to me that what is needed is a List or even a HashSet when replacing the "*" in the sql query with a "DISTINCT GroupName"

在我看来，需要的是一个列表，甚至是一个HashSet，当用一个“不同的GroupName”替换sql查询中的“*”时

So what exactly is returned when a single column from several records is returned? IOW, what should be within the angle brackets of the call to QueryAsync<>() ?

那么，当返回多个记录的单个列时，返回的究竟是什么?在调用QueryAsync<>()的尖括号内应该是什么?

I would think that this would work:

我认为这是可行的:

public async Task<List<String>> SelectDistinctGroupNames()
{
    var db = new SQLiteAsyncConnection(SQLitePath);
    List<string> allLocations = await db.QueryAsync<string>("SELECT DISTINCT GroupName FROM SOs_Locations ORDER BY GroupName");
    return allLocations;
}

...but with that I get, "'string' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'SQLite.SQLiteAsyncConnection.QueryAsync(string, params object[])'"

…但是，我得到的是，“string”必须是一个非抽象类型，使用公共无参数构造函数，以便在泛型类型或方法的SQLite.SQLiteAsyncConnection中使用它作为参数“T”。QueryAsync(字符串,params对象[])”

I had a "string" above to correspond with <SOs_Locations> (not "List<SOs_Locations>") in the working version of the method. When I change it to "List<string>", I get, "Cannot implicitly convert type 'System.Collections.Generic.List<System.Collections.Generic.List' to 'System.Collections.Generic.List<string>'"

在该方法的工作版本中，我有一个“字符串”，以对应于< sos_location >(不是“List< sos_location >”)。当我将其更改为“List ”时，我得到“不能隐式转换类型”System.Collections.Generic.List< system . collections.general。列表”到“System.Collections.Generic.List <字符串> ”

2 个解决方案

#1

From the source:

从源:

public Task<List<T>> QueryAsync<T> (string sql, params object[] args)
        where T : new ()

So if you want List<string> as a result, you would need to pass string for T. As you noticed, string cannot satisfy a new() constraint (because it does not have a parameterless constructor), so that's not possible.

因此，如果您想要列表，那么您就需要为t传递字符串。正如您所注意到的，string不能满足一个新的()约束(因为它没有参数化的构造函数)，所以这是不可能的。

I took a look through the code and it looks to me like the new() constraint is unnecessary, so my first stop would be with the sqlite-net folks asking them to remove it (and verifying that T=string will work).

我查看了代码，它看起来像新的()约束是不必要的，所以我的第一站是sqlite-net的人要求他们删除它(并验证T=string将工作)。

In the meantime, you should be able to create a type for the result of this query:

同时，您应该能够为这个查询创建一个类型:

public sealed class DistinctGroupNamesResult
{
  public string GroupName { get; set; }
}

and use a brief transformation:

并使用一个简短的转换:

public async Task<List<String>> SelectDistinctGroupNames()
{
  var db = new SQLiteAsyncConnection(SQLitePath);
  var result = await db.QueryAsync<DistinctGroupNamesResult>("SELECT DISTINCT GroupName FROM SOs_Locations");
  return result.Select(x => x.GroupName).ToList();
}

There may be simpler solutions, e.g., using the higher-level LINQ provider (not sure if it supports DISTINCT, though). Or it may be possible to use SOs_Locations as the result type instead of DistinctGroupNamesResult.

可能有更简单的解决方案，例如，使用更高级别的LINQ提供者(但不确定是否支持不同的)。或者，可以使用sos_location作为结果类型，而不是特殊的groupnamesresult。

#2

    List<String> resultString = new List<string>(); 
 var result = await db.QueryAsync<DistinctGroupNamesResult>("SELECT * FROM SOs_Locations");

       foreach (string item in result.Select(x=> x.GroupName).Distinct())
                {
                    resultString.Add(item);
                }
      return resultString;

#1