如何使用HttpClient从WebApi返回数据

Question

我一直没有成功地尝试从内部开发的WebApi返回数据。API使用Post请求返回包含复杂条件的请求体的数据。我以前曾成功地使用Post Request通过API写入数据，但我似乎在返回数据的努力中遇到了瓶颈。下面是我一直在使用的方法:

Public Async Function Request(ByVal client As HttpClient, ByVal content As String) As Task
Dim buffer = System.Text.Encoding.UTF8.GetBytes(content)
Dim byteContent = New ByteArrayContent(buffer)
byteContent.Headers.ContentType = New MediaTypeHeaderValue("application/json")
Dim response = Await client.PostAsync(client.BaseAddress, byteContent)
Dim result As String = response.Content.ReadAsStringAsync().Result
_result = result
End Function

为方便起见，结果存储在一个类变量中，这使我能够使用MSTest测试例程。这个函数应该返回一个JSON数组，但是它甚至不返回一个错误，而只是返回Nothing。

下面是相关的TestMethod:

<TestMethod()> Public Sub ShouldSuccessfullyInterrodateTheSpenHapi()
Dim da = New SPEN.ODSDataPackagerService
Dim crit As New HttpCriterion
Dim stopped As Boolean
Dim jsonString As String = Nothing
crit.StartTime = "2022-11-03T00:00:00"
crit.EndTime = "2022-11-03T00:30:00"
crit.Interval = "30m"
crit.TagAttribute = "InstrumentTag"
crit.TagFilter = "confidential"
crit.Delimiter = "~"
crit.ServerName = "SPRODA"
'Deserialize object to JSON
jsonString = da.GetCriteriaJson(crit)

Try
da.InitialiseSettings()
da.Request(da.Clients(0), jsonString)
Debug.Print(da.Result)
Assert.IsTrue(True)
Catch ex As Exception
Assert.IsTrue(False)
End Try
End Sub

我错过了什么?

编辑:

好了，用下面的代码做进一步的实验，我现在至少得到了一个错误:

Public Async Function GetVoltagesFromPI(ByVal client As HttpClient, ByVal content As String) As Task(Of PIItemList)
Dim piItems As PIItemList = New PIItemList
Dim buffer = System.Text.Encoding.UTF8.GetBytes(content)
Dim byteContent = New ByteArrayContent(buffer)
byteContent.Headers.ContentType = New MediaTypeHeaderValue("application/json")
Try
Dim response As New HttpResponseMessage
response = Await client.PostAsync(client.BaseAddress, byteContent)
If response.IsSuccessStatusCode Then
_result = New PIItemList
_result = JsonConvert.DeserializeObject(response.Content.ReadAsStringAsync().Result)
End If
Catch ex As Exception
Throw
Finally
End Try
End Function

函数错误在:

response = Await client.PostAsync(client.BaseAddress, byteContent)

:

System.NullReferenceException: 'Object variable or With block 

但我真的没有进一步的解决方案，因为我可以使代码应该工作，但有明显的东西在代码实现缺陷。

亲切的问候保罗。

Answer 1

最后…设法让事情工作，这里是基本的工作代码。如果有任何额外的意见，我将不胜感激:

''' <summary>
'''  Request pi data via the specified http endpoint (baseaddress)
''' </summary>
''' <param name="client"></param>
''' <param name="content"></param>
Public Function GetVoltagesFromPI(ByVal client As HttpClient, ByVal content As String) As Task(Of PIItemList)
Dim requestMsg As New HttpRequestMessage(HttpMethod.Post, client.BaseAddress)
Dim response As HttpResponseMessage
Dim interim As Object
Try
requestMsg.Content = New StringContent(content, Text.UTF8Encoding.Default, "application/json")
response = client.SendAsync(requestMsg).Result
If (response.StatusCode = System.Net.HttpStatusCode.OK) Then
_resultSet = JsonConvert.DeserializeObject(response.Content.ReadAsStringAsync().Result)
End If
Catch ex As Exception
Throw
Finally
End Try
End Function