[英]The type cannot be used as type parameter in the generic type or method
我有一個DBHelper類,可以從該類中觸發將IDataReader轉換為通用類型的回調。 我已經啟動並運行了整個代碼。 但是,當我嘗試在回調中創建該類型的實例並應用where子句時,會遇到麻煩(請參見類TestOneCallbacks中的-> DataReaderToResultItemConverterCallback)。
where子句使之完美。 TResult將為TestOne ResultItem類型。 但是,我無法使用where子句來完成這項工作。 另外,直接創建TestTwo ResultItem [請參見->類TestTwoCallbacks]並進行強制轉換可以使其正常工作,但並不美觀。
這是TestOneCallbacks的以下錯誤,
在通用類型或方法“ DataAccess.TestOneCallbacks.DataReaderToResultItemConverterCallback(System.Data.IDataReader)”中,類型“ TResult”不能用作類型參數“ TResult”。 沒有從“ TResult”到“ Service.Interface.TestFilterResultItem”的裝箱轉換或類型參數轉換。
這是代碼。
public static class DBHelper
{
public static List<TResult> ExecuteQuery<TResult>(DataQueryArgs<TResult> dataQueryArgs)
{
List<TResult> result = null;
{
IDataReader reader = null;
//Get the input parameters for the query
Dictionary<string, object> inputParameters = dataQueryArgs.InputParameters;
//Fire callback to Get the DbCommand for the query
DbCommand command = dataQueryArgs.GetCommandCallback();
//Fire callback to Get the list of Sql input Parameters for the query
List<SqlParameter> sqlParameters = dataQueryArgs.CreateSQLParametersCallback(inputParameters);
//Execute the query and get an IDataReader object
reader = ExecuteQuery(command, sqlParameters);
//Fire callback Convert the IReader to ResultItem
result = dataQueryArgs.DataReaderToResultItemConverterCallback(reader);
}
return result;
}
private static IDataReader ExecuteQuery(DbCommand command, List<SqlParameter> sqlParameters)
{
IDataReader reader = null;
{
//TODO:
}
return reader;
}
}
class TestOneResultItem
{
public TestOneResultItem()
{
}
public string Name { get; set; }
public int Age { get; set; }
public DateTime DOB { get; set; }
}
class TestTwoResultItem
{
public TestTwoResultItem()
{
}
public string Description { get; set; }
public int Size { get; set; }
}
public class TestOneCallbacks
{
public DataQueryArgs<TResult> InitializeDataQueryCallbacks<TResult>(Dictionary<string, object> dataQueryArgs_inputParameters)
{
DataQueryArgs<TResult> args = null;
DataQueryArgs<TResult>.DataReaderToResultItemConverter<TResult> dataReaderToResultItemConverterCallback = this.DataReaderToResultItemConverterCallback<TResult>;
args = new DataQueryArgs<TResult>(dataReaderToResultItemConverterCallback);
return args;
}
public List<TResult> DataReaderToResultItemConverterCallback<TResult>(IDataReader reader) where TResult : TestOneResultItem
{
List<TResult> results = new List<TResult>();
TResult t1 = new TResult(); //Activator.CreateInstance ???
{
t1.Name = "ShaQ";
t1.Age = 99;
t1.DOB = DateTime.Now;
}
results.Add(t1);
return results;
}
}
public class TestTwoCallbacks
{
public DataQueryArgs<TResult> InitializeDataQueryCallbacks<TResult>(Dictionary<string, object> dataQueryArgs_inputParameters)
{
DataQueryArgs<TResult> args = null;
DataQueryArgs<TResult>.DataReaderToResultItemConverter<TResult> dataReaderToResultItemConverterCallback = this.DataReaderToResultItemConverterCallback<TResult>;
args = new DataQueryArgs<TResult>(dataReaderToResultItemConverterCallback);
return args;
}
public List<TResult> DataReaderToResultItemConverterCallback<TResult>(IDataReader reader) where TResult : TestTwoResultItem
{
var results = new List<TestTwoResultItem>();
TestTwoResultItem t1 = new TestTwoResultItem(); //Activator.CreateInstance ???
{
t1.Description = "This works...";
t1.Size = 345;
}
results.Add(t1);
return results as List<TestTwoResultItem>; //Ugly, but I know exactly what I am doing. This cast will succeed!
}
}
如果將TResult
傳遞給該方法,則從調用方法派生的TResult
應具有與被調用方法相同的類型約束。 因此,您必須將where TResult : TestOneResultItem
的類型約束添加到調用方法: InitializeDataQueryCallbacks
。
public DataQueryArgs<TResult> InitializeDataQueryCallbacks<TResult>
(Dictionary<string, object> dataQueryArgs_inputParameters)
where TResult : TestOneResultItem
順便說一句,為了調用TResult
構造函數,您需要new()
約束:
where TResult : TestOneResultItem, new()
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