c# - 泛型参数的可空返回值

Question

我正在尝试使用 Dapper 将模型保存到数据库中。我使用输入/输出参数设置参数，该参数是一个 int，具有用于更新的现有主键值。

public async Task<TKey> SaveAsync<TKey>(IGraph builder, IDataContext context = null)
{
    var parameters = this.GetParametersFromDefinition(builder, DefinitionDirection.In);

    // See if we have a key defined. If not, we assume this is a new insert.
    // Otherwise we pull the key out and assume it's an update.
    PropertyDefinition key = builder.GetKey();

    if (key != null)
    {
        parameters.Add(key.ResolvedName, key.PropertyValue, null, ParameterDirection.InputOutput);
    }
    else
    {
        throw new InvalidOperationException("The data graph did not have a primary key defined for it.");
    }

    await this.ExecuteProcedure(parameters, builder, context);

    object returnedId = parameters.Get<TKey>(key.ResolvedName);
    return returnedId == null ? default(TKey) : (TKey)returnedId;
}

private Task ExecuteProcedure(DynamicParameters parameters, IGraph builder, IDataContext context = null)
{
    ProcedureDefinition mapping = builder.GetProcedureForOperation(ProcedureOperationType.Insert);
    if (string.IsNullOrEmpty(mapping.StoredProcedure))
    {
        throw new InvalidOperationException("No stored procedure mapped to the builder.");
    }

    // Query the database
    return this.SetupConnection(
        context,
        (connection, transaction) => connection.ExecuteAsync(
            mapping.StoredProcedure,
            parameters,
            commandType: CommandType.StoredProcedure,
            transaction: transaction));
}

它是这样调用的:

this.Address.AddressId = await repository.SaveAsync<int>(graph);

当我评估参数时，我会在上面看到我的输入/输出参数。

然而，当我尝试在我的保存中执行这一行时:

TKey returnedId = parameters.Get<TKey>(key.ResolvedName);

我得到以下异常:

Exception:Caught: "Attempting to cast a DBNull to a non nullable type! Note that out/return parameters will not have updated values until the data stream completes (after the 'foreach' for Query(..., buffered: false), or after the GridReader has been disposed for QueryMultiple)" (System.ApplicationException) A System.ApplicationException was caught: "Attempting to cast a DBNull to a non nullable type! Note that out/return parameters will not have updated values until the data stream completes (after the 'foreach' for Query(..., buffered: false), or after the GridReader has been disposed for QueryMultiple)" Time: 7/21/2015 10:19:48 PM Thread:[7200]

我假设这是一个问题，因为泛型类型在这种情况下不能为空，因为它是一个整数。这是因为 Dapper 总是返回可空值吗？我刚刚暂时为存储过程的 OUTPUT 分配了一个常量值，以确保为输出分配了一些东西。

我如何解决返回 nullable 的 dapper，唯一的解决方法是将 int? 作为泛型类型传递吗？

更新

我能够使用这种方法解决它。所以现在还不允许我将其作为答案发布。当时间限制到时，我会发布一个答案，除非其他人有更好的想法。

object returnedId = parameters.Get<TKey>(key.ResolvedName);
if (returnedId == null)
{
    return default(TKey);
}

return (TKey)returnedId;

Answer 1

如果 DBNull 是一个有效值并且应该表示除 default(TKey) 之外的其他含义(例如:default(int) = 0)，并且如果 TKey 将始终是一个值类型，则将 TKey 约束为结构，如下所示:

public async Task<TKey?> SaveAsync<TKey>(IGraph builder, IDataContext context = null) where TKey : struct
{
    ...
}

然后像这样获取 key :

TKey? returnedId = parameters.Get<TKey?>(key.ResolvedName);

SaveAsync 的返回类型将反射(reflect)键可能为空。如果键永远不应该为 null，并且如果 DBNull 应该默认为 default(TKey)，则只需使用以下内容:

public async Task<TKey?> SaveAsync<TKey>(IGraph builder, IDataContext context = null) where TKey : struct
{
    ...
    return parameters.Get<TKey?>(key.ResolvedName).GetValueOrDefault();
}