I’ve been working on a tool lately to parse a bunch of work item data from Azure DevOps. Specifically, I needed to call the Work Item Updates REST service in order to get all the state value changes for a bunch of work items.
Everything was fine until I needed to convert the JSON result data from strings into objects. Then I started getting tons of exceptions saying things like “System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $.value[0].fields[‘System.Id’].newValue | LineNumber: 24 | BytePositionInLine: 25. —> System.InvalidOperationException: Cannot get the value of a token type ‘Number’ as a string.”
The ultimate answer was a custom implementation of JsonConverter<T>. (If you’re feeling impatient, here’s the sample code.)
Strange Data Formats
In the return value from that service, there’s a collection of changed field values that gives you the old value and the new value as those fields change. Well, in Azure DevOps, a field can be a handful of different data types including integer, string, boolean, date, or a person’s identity. Check out the JSON snippet below. The fields property is basically a Dictionary<string, object>. The keys are values like “System.Rev” or “System.State” but then the values for the “oldValue” and “newValue” properties can be completely different. For example, the data type for System.Rev is a number, the value for System.State is a string, System.ChangedDate is a datetime, and then Microsoft.VSTS.Common.ClosedBy is a really complex object that represents the person who performed the action.
"fields": {
"System.Rev": {
"oldValue": 5,
"newValue": 6
},
"System.State": {
"oldValue": "Active",
"newValue": "Closed"
},
"System.ChangedDate": {
"oldValue": "2022-08-19T12:39:33.07Z",
"newValue": "2022-08-19T12:39:37.47Z"
},
"Microsoft.VSTS.Common.ClosedBy": {
"newValue": {
"displayName": "Benjamin Day",
"url": "https://spsprodeus22.vssps.visualstudio.com/B5q33db6d-a202-4c3a-ae2c-be1d2a3875c3/_apis/Identities/B5q33db6d-7f35-4df7-b308-395bf4eaf8d9",
"_links": {
"avatar": {
"href": "https://dev.azure.com/benday/_apis/GraphProfile/MemberAvatars/aad.mmmmmWFkZTYtOWI1ZS03NTZlLWExblatZTU2NDIbla3MzYw"
}
},
"id": "B5q33db6d-7f35-4df7-b308-395bf4eaf8d9",
"uniqueName": "benday@benday.com",
"imageUrl": "https://dev.azure.com/benday/_apis/GraphProfile/MemberAvatars/aad.mmmmmWFkZTYtOWI1ZS03NTZlLWExblatZTU2NDIbla3MzYw",
"descriptor": "aad.mmmmmWFkZTYtOWI1ZS03NTZlLWExblatZTU2NDIbla3MzYw"
}
}
}System.Text.Json.JsonSerializer Gets Confused
In my application, I just needed to access the oldValue and newValue properties as strings. When the JsonSerializer was asked to parse that fields property as Dictionary<string, object>, everything was fine because the deserialization wasn’t especially worried about types. Can it be converted to an object? Yes. Good. Move on.
For example, if I tried to deserialize that fields data into a class like WorkItemRevisionInfoWithFieldsAsObjects (see below), it’s fine.
public class WorkItemRevisionInfoWithFieldsAsObjects
{
[JsonPropertyName("fields")]
public Dictionary<string, object> Fields { get; set; } = new();
// ...
} But when I tried to change that to use Dictionary<string, FieldRevision>, then it the problems started.
public class WorkItemRevisionInfoWithFieldsAsTypedValues
{
// [JsonPropertyName("fields")]
// public Dictionary<string, object> Fields { get; set; }
[JsonPropertyName("fields")]
public Dictionary<string, FieldRevision> Fields { get; set; } = new();
// ...
}The FieldRevision class wasn’t all that complex either — just two string properties.
public class FieldRevision
{
[JsonPropertyName("oldValue")]
public string OldValue { get; set; } = String.Empty;
[JsonPropertyName("newValue")]
public string NewValue { get; set; } = String.Empty;
}But once I started trying to deserialize that JSON, there were errors all over the place.
Unhandled exception. System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $.value[0].fields['System.Id'].newValue | LineNumber: 24 | BytePositionInLine: 25. ---> System.InvalidOperationException: Cannot get the value of a token type 'Number' as a string. at System.Text.Json.Utf8JsonReader.GetString() at System.Text.Json.Serialization.Converters.StringConverter.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options) at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader) at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonDictionaryConverter`3.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TDictionary& value) at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader) at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonCollectionConverter`2.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TCollection& value) at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader) at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state) --- End of inner exception stack trace --- at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& state, Utf8JsonReader& reader, Exception ex) at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state) at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo jsonTypeInfo, Nullable`1 actualByteCount) at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo jsonTypeInfo) at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options) at Program.<<Main>$>g__ParseUsingTypedValues|0_0(String json) in /Users/benday/code/temp/Benday.JsonConverterSample/Benday.JsonConverterSample/Program.cs:line 12 at Program.<Main>$(String[] args) in /Users/benday/code/temp/Benday.JsonConverterSample/Benday.JsonConverterSample/Program.cs:line 8
JsonConverter<T> to the Rescue
The solution is to add an implementation of JsonConverter<T> to my project and then to instruct the JsonSerializer to use that converter to deserialize those properties.
Here’s my implementation:
public class EverythingToStringJsonConverter : JsonConverter<string>
{
public override string Read(ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
if (reader.TokenType == JsonTokenType.String)
{
return reader.GetString() ?? String.Empty;
}
else if (reader.TokenType == JsonTokenType.Number)
{
var stringValue = reader.GetDouble();
return stringValue.ToString();
}
else if (reader.TokenType == JsonTokenType.False ||
reader.TokenType == JsonTokenType.True)
{
return reader.GetBoolean().ToString();
}
else if (reader.TokenType == JsonTokenType.StartObject)
{
reader.Skip();
return "(not supported)";
}
else
{
Console.WriteLine($"Unsupported token type: {reader.TokenType}");
throw new System.Text.Json.JsonException();
}
}
public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
{
writer.WriteStringValue(value);
}
}Once I had that class in my project, I just had to tell the JsonSerializer where to use it. In this case, I needed to add a [JsonConverter] attribute to the properties on my FieldRevisions class.
public class FieldRevision
{
[JsonPropertyName("oldValue")]
[JsonConverter(typeof(EverythingToStringJsonConverter))]
public string OldValue { get; set; } = String.Empty;
[JsonPropertyName("newValue")]
[JsonConverter(typeof(EverythingToStringJsonConverter))]
public string NewValue { get; set; } = String.Empty;
}After that, everything ran fine and I had the data I needed in the format that I wanted.
Summary
When you need to make custom adjustments when you serializing and deserializing your JSON data to or from objects, you’ll want to create an implementation of JsonConverter<T>. Create an implementation of JsonConverter and then add the JsonConverter attribute to you the properties and classes where you need that custom conversion.
I hope this helps.
-Ben
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