为什么不能在c#中将null转换为类型参数T？

Public Function FindItem(ByVal p_propertyName As String,ByVal p_value As Object) As T

    Dim index As Int32

    index = FindIndex(p_propertyName,p_value)

    If index >= 0 Then
        Return Me(index)
    End If

    Return Nothing

End Function

它允许为T返回Nothing(null)

C#等价物不起作用：

public T FindItem(string p_propertyName,object p_value)
{
  Int32 index = FindIndex(p_propertyName,p_value);

  if (index >= 0) {
    return this[index];
  }
  return null;
}

它不会使用此错误进行编译：

The type ‘T’ must be a non-nullable value type in order to use it as parameter ‘T’ in the generic type or method 'System.Nullable<T>'

我需要能够具有相同的功能,否则会破坏很多代码.我错过了什么？