问题
Using VB.NET, I'm trying to clean up a code base following ReSharper's guidelines. I currently have the following code:
'oSearchInput is defined outside this question
Dim oSearchRoutines As New SearchClient
Dim oSearchResults As List(Of SearchResult)
oSearchRoutines = 'WcfCallThatReturnsSearchClient
oSearchResults = oSearchRoutines.getSearchResults(oSearchInput).ToList
Now this works completely fine, but ReSharper warns that As New SearchClient has 'Value assigned is not used in any execution path'. So I removed that part to get this code:
'oSearchInput is defined outside this question
Dim oSearchRoutines
Dim oSearchResults As List(Of SearchResult)
oSearchRoutines = 'WcfCallThatReturnsSearchClient
oSearchResults = oSearchRoutines.getSearchResults(oSearchInput).ToList
If I'm understanding this correctly, everything should work exactly the same. However, an error is thrown when calling ToList:
Public member 'ToList' on type 'SearchResult()' not found.
I'm not exactly sure why there's any difference between the two snippets I have here.
回答1:
Because you're not assinging the type SearchClient in your second example, oSearchRoutines will automatically be of type Object.
An expression of type Object is mainly not allowed to use Extension methods, like for example the ToList-method. For further information see here
The following example illustrates this behaviour:
Dim x As Object
Dim y As String = "ABC"
x = y
Dim a As List(Of Char) = y.ToList() 'This will work
Dim b As List(Of Char) = x.ToList() 'This will throw a System.MissingMemberException
The message Value assigned is not used in any execution path, appears because you're declaring oSearchRoutines with New in your first example.
This is unnecessary because you're assinging a new value to it on the line...
oSearchRoutines = 'WcfCallThatReturnsSearchClient
...before using it anywhere.
Thus you can just declare it without the keyword New
Dim oSearchRoutines As SearchClient
Related question: VB.NET: impossible to use Extension method on System.Object instance
来源：https://stackoverflow.com/questions/47657062/implicitly-defined-variable-throws-run-time-error-while-explicitly-defined-does