VB.NET Requires Chilkat v11.7.0+
Box: Upload a Local File with HttpCurl

Run the reusable setup once per HttpCurl object, then select names and execute the operation. The setup is repeated here only to make this published example independently runnable. Upload a local PDF into documents, selected by name. Resolve the folder with the registry before building multipart attributes. This explicit second phase avoids literal placeholders in -F fields; files are limited to 50 MB for direct uploads.
See the Box API guide for setup and the complete workflow.
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Dim success As Boolean = False

'  Requires Chilkat 11.7.0 or later and a previously unlocked Chilkat bundle.
'  See Global Unlock Sample.
'  Guide: Box API Guide.

success = False

'  BEGIN ONE-TIME SETUP
'  This block is repeated so each published example can run on its own.
'  In an application, run the setup example once and pass the SAME curl object
'  to operation code below. Do not recreate it for every operation.
'  See Reusable Box HttpCurl Setup.
Dim curl As New Chilkat.HttpCurl
curl.SetVar("access_token","BOX_ACCESS_TOKEN")
curl.SetVar("parent_name","/")
'  Box's fixed root ID is an implementation detail; callers supply names.
curl.SetVar("parent_id","0")
Dim setupCommand As New Chilkat.StringBuilder
Dim setupReplacements As Integer

'  Register findParent; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("findParent",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register findFolder; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/folders/{{parent_id}}/items?limit=1000&fields=id,type,name"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("findFolder",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register findFile; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/folders/{{folder_id}}/items?limit=1000&fields=id,type,name"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("findFile",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register findDestination; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("findDestination",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register getFileDetails; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/files/{{file_id}}?fields=id,sha1,file_version"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("getFileDetails",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register getEnterprise; registration sends no request.
setupCommand.SetString("curl -X GET ""https://api.box.com/2.0/users/me?fields=enterprise"" -H ""Authorization: Bearer ACCESS_TOKEN""")
setupReplacements = setupCommand.Replace("ACCESS_TOKEN",curl.GetVar("access_token"))
success = curl.AddFunction("getEnterprise",setupCommand.GetAsString())
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If


'  Register output rules once. Chilkat 11.7.0 evaluates {{name}} variables
'  when extracting each output, using the current value at that time.
success = curl.AddOutput2("findParent","entries","name","{{parent_name}}",True,"id","parent_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput2("findFolder","entries","name","{{folder_name}}",True,"id","folder_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput2("findFile","entries","name","{{file_name}}",True,"id","file_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput2("findDestination","entries","name","{{destination_name}}",True,"id","destination_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput("getFileDetails","file_version.id","file_version_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput("getFileDetails","sha1","file_sha1")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

success = curl.AddOutput("getEnterprise","enterprise.id","enterprise_id")
If (success <> True) Then
    Debug.WriteLine(curl.LastErrorText)
    Exit Sub
End If

'  END ONE-TIME SETUP
'  The name lookups below assume each listing fits in one page (1000 items).
'  Parent and destination names select root children; folder_name selects a child
'  of parent_name. "/" selects the root parent. These are not arbitrary paths.

'  BEGIN OPERATION -- keep the configured object when reusing it.
'  Inputs are names. Retain cached IDs when the selected names have not changed.
Dim parentName As String = "/"
Dim folderName As String = "documents"
If ((curl.GetVar("parent_name") <> parentName) Or ((parentName = "/") And (curl.VarDefined("parent_id") = False))) Then
    curl.SetVar("parent_name",parentName)
    curl.ClearVar("parent_id")
    curl.ClearVar("folder_id")
    curl.ClearVar("file_id")
    curl.ClearVar("file_version_id")
    curl.ClearVar("file_sha1")
    If (parentName = "/") Then
        curl.SetVar("parent_id","0")
    End If

End If

If (curl.GetVar("folder_name") <> folderName) Then
    curl.SetVar("folder_name",folderName)
    curl.ClearVar("folder_id")
    curl.ClearVar("file_id")
    curl.ClearVar("file_version_id")
    curl.ClearVar("file_sha1")
End If

'  Resolve the named destination first. HttpCurl does not expand variables
'  inside multipart -F attributes in the tested build, so this upload is two phases.
curl.ClearTargetOutput("*")
Dim authorizedCommand1 As New Chilkat.StringBuilder
authorizedCommand1.SetString("curl -X GET ""https://api.box.com/2.0/folders/{{folder_id}}?fields=id"" -H ""Authorization: Bearer {{access_token}}""")
Dim headerReplacement1 As Integer = authorizedCommand1.Replace("{{access_token}}",curl.GetVar("access_token"))
success = curl.DoYourThing(authorizedCommand1.GetAsString())
If ((success <> True) Or (curl.StatusCode <> 200)) Then
    Debug.WriteLine("Box request failed. HTTP status: " & curl.StatusCode)
    Debug.WriteLine("Failure reason: " & curl.FailReason)
    Debug.WriteLine(curl.ResponseBodyStr)
    Exit Sub
End If

'  The file is read from the path following @. Keep attributes before file.
'  Resolve the multipart attributes field before passing the command to HttpCurl.
'  Box folder IDs come from configuration or a previous API response.
Dim uploadCommand As New Chilkat.StringBuilder
Dim authorizedCommand2 As New Chilkat.StringBuilder
authorizedCommand2.SetString("curl -X POST ""https://upload.box.com/api/2.0/files/content"" -H ""Authorization: Bearer {{access_token}}"" -F 'attributes={""name"":""invoice.pdf"",""parent"":{""id"":""{{folder_id}}""}}' -F ""file=@qa_data/pdf/invoice.pdf;type=application/pdf""")
Dim headerReplacement2 As Integer = authorizedCommand2.Replace("{{access_token}}",curl.GetVar("access_token"))
uploadCommand.SetString(authorizedCommand2.GetAsString())
Dim replacements As Integer = uploadCommand.Replace("{{folder_id}}",curl.GetVar("folder_id"))
curl.ClearTargetOutput("*")
curl.ClearVar("file_id")
curl.AddTargetOutput("entries[0].id","file_id")
success = curl.DoYourThing(uploadCommand.GetAsString())
If ((success <> True) Or (curl.StatusCode <> 201)) Then
    Debug.WriteLine("Box request failed. HTTP status: " & curl.StatusCode)
    Debug.WriteLine("Failure reason: " & curl.FailReason)
    Debug.WriteLine(curl.ResponseBodyStr)
    Exit Sub
End If

If (curl.VarDefined("!") <> True) Then
    Debug.WriteLine("Expected response fields are missing.")
    Exit Sub
End If

Debug.WriteLine("Uploaded file ID: " & curl.GetVar("file_id"))
curl.SetVar("file_name","invoice.pdf")
curl.ClearVar("file_version_id")
curl.ClearVar("file_sha1")