问题
I need to obtain the "title" from the response i get from Youtube. I get the following error at line 8, position 12, that is, at line:
"items": [
just after the "["
The error i get is:
Exception: Exception thrown: 'Newtonsoft.Json.JsonSerializationException' in Newtonsoft.Json.dll ("Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'categoryid.Item' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List<T> that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.
Path 'items', line 8, position 12."). Exception thrown: 'Newtonsoft.Json.JsonSerializationException' in Newtonsoft.Json.dll ("Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'categoryid.Item' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List<T> that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.
Path 'items', line 8, position 12.") 2.61s [12096] <No Name>
I have this code:
Dim m As IEnumerable(Of Rootobject) = JsonConvert.DeserializeObject(Of IEnumerable(Of Rootobject))(res)
with this JSON data:
{
"kind":"youtube#videoListResponse",
"etag":"\"m2yskBQFythfE4irbTIeOgYYfBU/jznkhy3_Aws9VtommTkcdOYnAAk\"",
"pageInfo":{
"totalResults":1,
"resultsPerPage":1
},
"items":[
{
"kind":"youtube#video",
"etag":"\"m2yskBQFythfE4irbTIeOgYYfBU/M_wQmC4lQBaHJGxo79N7WlmqNr8\"",
"id":"SSbBvKaM6sk",
"snippet":{
"publishedAt":"2009-04-15T20:31:11.000Z",
"channelId":"UC2kTZB_yeYgdAg4wP2tEryA",
"title":"Blur - Song 2",
"description":"Blur 21 -- Celebrating 21 years of Blur. To find out more, click here:http://smarturl.it/blur21y\n\n#blur21\n \nFollow Blur on Twitter:www.twitter.com/blurofficial \nFind Blur on Facebook:www.facebook.com/blur\n\nMusic video by Blur performing Song 2.",
"thumbnails":{
"default":{
"url":"https://i.ytimg.com/vi/SSbBvKaM6sk/default.jpg",
"width":120,
"height":90
},
"medium":{
"url":"https://i.ytimg.com/vi/SSbBvKaM6sk/mqdefault.jpg",
"width":320,
"height":180
},
"high":{
"url":"https://i.ytimg.com/vi/SSbBvKaM6sk/hqdefault.jpg",
"width":480,
"height":360
},
"standard":{
"url":"https://i.ytimg.com/vi/SSbBvKaM6sk/sddefault.jpg",
"width":640,
"height":480
},
"maxres":{
"url":"https://i.ytimg.com/vi/SSbBvKaM6sk/maxresdefault.jpg",
"width":1280,
"height":720
}
},
"channelTitle":"emimusic",
"tags":[
"Blur",
"Song"
],
"categoryId":"10",
"liveBroadcastContent":"none",
"localized":{
"title":"Blur - Song 2",
"description":"Blur 21 -- Celebrating 21 years of Blur. To find out more, click here:http://smarturl.it/blur21y\n\n#blur21\n \nFollow Blur on Twitter:www.twitter.com/blurofficial \nFind Blur on Facebook:www.facebook.com/blur\n\nMusic video by Blur performing Song 2."
}
},
"contentDetails":{
"duration":"PT2M3S",
"dimension":"2d",
"definition":"sd",
"caption":"false",
"licensedContent":true,
"regionRestriction":{
"allowed":[
"BY",
"US"
]
},
"projection":"rectangular"
}
}
]
}
and these classes. the classes are pasted as special with the visual studio 2015 option.
Public Class Rootobject
Public Property kind As String
Public Property etag As String
Public Property pageInfo As Pageinfo
Public Property items() As Item
End Class
Public Class Pageinfo
Public Property totalResults As Integer
Public Property resultsPerPage As Integer
End Class
Public Class Item
Public Property kind As String
Public Property etag As String
Public Property id As String
Public Property snippet As Snippet
Public Property contentDetails As Contentdetails
End Class
Public Class Snippet
Public Property publishedAt As Date
Public Property channelId As String
Public Property title As String
Public Property description As String
Public Property thumbnails As Thumbnails
Public Property channelTitle As String
Public Property tags() As String
Public Property categoryId As String
Public Property liveBroadcastContent As String
Public Property localized As Localized
End Class
Public Class Thumbnails
Public Property _default As _Default
Public Property medium As Medium
Public Property high As High
Public Property standard As Standard
Public Property maxres As Maxres
End Class
Public Class _Default
Public Property url As String
Public Property width As Integer
Public Property height As Integer
End Class
Public Class Medium
Public Property url As String
Public Property width As Integer
Public Property height As Integer
End Class
Public Class High
Public Property url As String
Public Property width As Integer
Public Property height As Integer
End Class
Public Class Standard
Public Property url As String
Public Property width As Integer
Public Property height As Integer
End Class
Public Class Maxres
Public Property url As String
Public Property width As Integer
Public Property height As Integer
End Class
Public Class Localized
Public Property title As String
Public Property description As String
End Class
Public Class Contentdetails
Public Property duration As String
Public Property dimension As String
Public Property definition As String
Public Property caption As String
Public Property licensedContent As Boolean
Public Property regionRestriction As Regionrestriction
Public Property projection As String
End Class
Public Class Regionrestriction
Public Property allowed() As String
End Class
What i need to do? Thanks in advance.
回答1:
You are getting that exception because you have declared several auto-implemented array properties incorrectly. As shown in Auto-Implemented Properties (Visual Basic) such properties should be declared as follows, with the () array indicator as part of the return type:
Public Property items As Item()
Instead you declare them as follows:
Public Property items() As Item
This declares a property returning a single Item rather than an array of them. The () attached to the property name is optional; for parameterless properties it is redundant but for properties taking parameters the argument list appears there. See the documentation page Property Statement for details. Then Json.NET throws the exception you see when attempting to deserialize a JSON array into one of the non-array properties in your model.
To fix this, three of your classes should be modified as follows:
Public Class Rootobject
Public Property kind As String
Public Property etag As String
Public Property pageInfo As Pageinfo
Public Property items As Item() ' Fixed
End Class
Public Class Snippet
Public Property publishedAt As Date
Public Property channelId As String
Public Property title As String
Public Property description As String
Public Property thumbnails As Thumbnails
Public Property channelTitle As String
Public Property tags As String() ' Fixed
Public Property categoryId As String
Public Property liveBroadcastContent As String
Public Property localized As Localized
End Class
Public Class Regionrestriction
Public Property allowed As String() ' Fixed
End Class
Then, since the items are an array, to access the titles and put then in a list, you can use extension methods from System.Linq.Enumerable:
' Convert the json string to RootObject.
Dim root = JsonConvert.DeserializeObject(Of Rootobject)(json)
' Extract the list of titles.
Dim titles = root.items _
.Select(Function(i) i.snippet.title) _
.ToList()
' Get the first title in the list.
Dim firstTitle = titles.FirstOrDefault()
Console.WriteLine("First title = ""{0}""", firstTitle)
Which prints out First title = "Blur - Song 2".
Sample VB.Net fiddle.
来源：https://stackoverflow.com/questions/45387913/cannot-json-deserialize-youtube-data-with-vb-net