asp.net – InvalidOperationException Nullable对象必须具有值

从MSDN page on nullable types

The Value property returns a value if one is assigned,otherwise a
System.InvalidOperationException is thrown.

您有多种选择来克服错误.例如：

int? a=null; // a test nullable object
//Console.WriteLine(a.Value); // this throws an InvalidOperationException

// using GetValueOrDefault()
Console.WriteLine(a.GetValueOrDefault()); //0 (default value for int)

//checking if a.HasValue
if(a.HasValue) Console.WriteLine(a.Value); // does not print anything as the if
                                           // is false

// using the ?? operator
Console.WriteLine(a ?? -1); // prints -1