.Net Core 3.0 JsonSerializer populate existing object
See the question and my original answer on StackOverflowHere is some sample code that does it. It's using the new Utf8JsonReader struct so it populates the object at the same time it parses it. It supports JSON/CLR types equivalence, nested objects (creates if they don't exist), lists and arrays.
var populator = new JsonPopulator();
var obj = new MyClass();
populator.PopulateObject(obj, "{\"Title\":\"Startpage\",\"Link\":\"/index\"}");
populator.PopulateObject(obj, "{\"Head\":\"Latest news\",\"Link\":\"/news\"}");
public class MyClass
{
public string Title { get; set; }
public string Head { get; set; }
public string Link { get; set; }
}
Note it doesn't support all of what you would probably expect, but you can override or customize it. Things that could be added: 1) naming convention. You'd have to override the GetProperty method. 2) dictionaries or expando objects. 3) performance can be improved because it uses Reflection instead of MemberAccessor/delegate techniques
public class JsonPopulator
{
public void PopulateObject(object obj, string jsonString, JsonSerializerOptions options = null) => PopulateObject(obj, jsonString != null ? Encoding.UTF8.GetBytes(jsonString) : null, options);
public virtual void PopulateObject(object obj, ReadOnlySpan<byte> jsonData, JsonSerializerOptions options = null)
{
options ??= new JsonSerializerOptions();
var state = new JsonReaderState(new JsonReaderOptions { AllowTrailingCommas = options.AllowTrailingCommas, CommentHandling = options.ReadCommentHandling, MaxDepth = options.MaxDepth });
var reader = new Utf8JsonReader(jsonData, isFinalBlock: true, state);
new Worker(this, reader, obj, options);
}
protected virtual PropertyInfo GetProperty(ref Utf8JsonReader reader, JsonSerializerOptions options, object obj, string propertyName)
{
if (obj == null)
throw new ArgumentNullException(nameof(obj));
if (propertyName == null)
throw new ArgumentNullException(nameof(propertyName));
var prop = obj.GetType().GetProperty(propertyName);
return prop;
}
protected virtual bool SetPropertyValue(ref Utf8JsonReader reader, JsonSerializerOptions options, object obj, string propertyName)
{
if (obj == null)
throw new ArgumentNullException(nameof(obj));
if (propertyName == null)
throw new ArgumentNullException(nameof(propertyName));
var prop = GetProperty(ref reader, options, obj, propertyName);
if (prop == null)
return false;
if (!TryReadPropertyValue(ref reader, options, prop.PropertyType, out var value))
return false;
prop.SetValue(obj, value);
return true;
}
protected virtual bool TryReadPropertyValue(ref Utf8JsonReader reader, JsonSerializerOptions options, Type propertyType, out object value)
{
if (propertyType == null)
throw new ArgumentNullException(nameof(reader));
if (reader.TokenType == JsonTokenType.Null)
{
value = null;
return !propertyType.IsValueType || Nullable.GetUnderlyingType(propertyType) != null;
}
if (propertyType == typeof(object)) { value = ReadValue(ref reader); return true; }
if (propertyType == typeof(string)) { value = JsonSerializer.Deserialize<JsonElement>(ref reader, options).GetString(); return true; }
if (propertyType == typeof(int) && reader.TryGetInt32(out var i32)) { value = i32; return true; }
if (propertyType == typeof(long) && reader.TryGetInt64(out var i64)) { value = i64; return true; }
if (propertyType == typeof(DateTime) && reader.TryGetDateTime(out var dt)) { value = dt; return true; }
if (propertyType == typeof(DateTimeOffset) && reader.TryGetDateTimeOffset(out var dto)) { value = dto; return true; }
if (propertyType == typeof(Guid) && reader.TryGetGuid(out var guid)) { value = guid; return true; }
if (propertyType == typeof(decimal) && reader.TryGetDecimal(out var dec)) { value = dec; return true; }
if (propertyType == typeof(double) && reader.TryGetDouble(out var dbl)) { value = dbl; return true; }
if (propertyType == typeof(float) && reader.TryGetSingle(out var sgl)) { value = sgl; return true; }
if (propertyType == typeof(uint) && reader.TryGetUInt32(out var ui32)) { value = ui32; return true; }
if (propertyType == typeof(ulong) && reader.TryGetUInt64(out var ui64)) { value = ui64; return true; }
if (propertyType == typeof(byte[]) && reader.TryGetBytesFromBase64(out var bytes)) { value = bytes; return true; }
if (propertyType == typeof(bool))
{
if (reader.TokenType == JsonTokenType.False || reader.TokenType == JsonTokenType.True)
{
value = reader.GetBoolean();
return true;
}
}
// fallback here
return TryConvertValue(ref reader, propertyType, out value);
}
protected virtual object ReadValue(ref Utf8JsonReader reader)
{
switch (reader.TokenType)
{
case JsonTokenType.False: return false;
case JsonTokenType.True: return true;
case JsonTokenType.Null: return null;
case JsonTokenType.String: return reader.GetString();
case JsonTokenType.Number: // is there a better way?
if (reader.TryGetInt32(out var i32))
return i32;
if (reader.TryGetInt64(out var i64))
return i64;
if (reader.TryGetUInt64(out var ui64)) // uint is already handled by i64
return ui64;
if (reader.TryGetSingle(out var sgl))
return sgl;
if (reader.TryGetDouble(out var dbl))
return dbl;
if (reader.TryGetDecimal(out var dec))
return dec;
break;
}
throw new NotSupportedException();
}
// we're here when json types & property types don't match exactly
protected virtual bool TryConvertValue(ref Utf8JsonReader reader, Type propertyType, out object value)
{
if (propertyType == null)
throw new ArgumentNullException(nameof(reader));
if (propertyType == typeof(bool))
{
if (reader.TryGetInt64(out var i64)) // one size fits all
{
value = i64 != 0;
return true;
}
}
// TODO: add other conversions
value = null;
return false;
}
protected virtual object CreateInstance(ref Utf8JsonReader reader, Type propertyType)
{
if (propertyType.GetConstructor(Type.EmptyTypes) == null)
return null;
// TODO: handle custom instance creation
try
{
return Activator.CreateInstance(propertyType);
}
catch
{
// swallow
return null;
}
}
private class Worker
{
private readonly Stack<WorkerProperty> _properties = new Stack<WorkerProperty>();
private readonly Stack<object> _objects = new Stack<object>();
public Worker(JsonPopulator populator, Utf8JsonReader reader, object obj, JsonSerializerOptions options)
{
_objects.Push(obj);
WorkerProperty prop;
WorkerProperty peek;
while (reader.Read())
{
switch (reader.TokenType)
{
case JsonTokenType.PropertyName:
prop = new WorkerProperty();
prop.PropertyName = Encoding.UTF8.GetString(reader.ValueSpan);
_properties.Push(prop);
break;
case JsonTokenType.StartObject:
case JsonTokenType.StartArray:
if (_properties.Count > 0)
{
object child = null;
var parent = _objects.Peek();
PropertyInfo pi = null;
if (parent != null)
{
pi = populator.GetProperty(ref reader, options, parent, _properties.Peek().PropertyName);
if (pi != null)
{
child = pi.GetValue(parent); // mimic ObjectCreationHandling.Auto
if (child == null && pi.CanWrite)
{
if (reader.TokenType == JsonTokenType.StartArray)
{
if (!typeof(IList).IsAssignableFrom(pi.PropertyType))
break; // don't create if we can't handle it
}
if (reader.TokenType == JsonTokenType.StartArray && pi.PropertyType.IsArray)
{
child = Activator.CreateInstance(typeof(List<>).MakeGenericType(pi.PropertyType.GetElementType())); // we can't add to arrays...
}
else
{
child = populator.CreateInstance(ref reader, pi.PropertyType);
if (child != null)
{
pi.SetValue(parent, child);
}
}
}
}
}
if (reader.TokenType == JsonTokenType.StartObject)
{
_objects.Push(child);
}
else if (child != null) // StartArray
{
peek = _properties.Peek();
peek.IsArray = pi.PropertyType.IsArray;
peek.List = (IList)child;
peek.ListPropertyType = GetListElementType(child.GetType());
peek.ArrayPropertyInfo = pi;
}
}
break;
case JsonTokenType.EndObject:
_objects.Pop();
if (_properties.Count > 0)
{
_properties.Pop();
}
break;
case JsonTokenType.EndArray:
if (_properties.Count > 0)
{
prop = _properties.Pop();
if (prop.IsArray)
{
var array = Array.CreateInstance(GetListElementType(prop.ArrayPropertyInfo.PropertyType), prop.List.Count); // array is finished, convert list into a real array
prop.List.CopyTo(array, 0);
prop.ArrayPropertyInfo.SetValue(_objects.Peek(), array);
}
}
break;
case JsonTokenType.False:
case JsonTokenType.Null:
case JsonTokenType.Number:
case JsonTokenType.String:
case JsonTokenType.True:
peek = _properties.Peek();
if (peek.List != null)
{
if (populator.TryReadPropertyValue(ref reader, options, peek.ListPropertyType, out var item))
{
peek.List.Add(item);
}
break;
}
prop = _properties.Pop();
var current = _objects.Peek();
if (current != null)
{
populator.SetPropertyValue(ref reader, options, current, prop.PropertyName);
}
break;
}
}
}
private static Type GetListElementType(Type type)
{
if (type.IsArray)
return type.GetElementType();
foreach (Type iface in type.GetInterfaces())
{
if (!iface.IsGenericType) continue;
if (iface.GetGenericTypeDefinition() == typeof(IDictionary<,>)) return iface.GetGenericArguments()[1];
if (iface.GetGenericTypeDefinition() == typeof(IList<>)) return iface.GetGenericArguments()[0];
if (iface.GetGenericTypeDefinition() == typeof(ICollection<>)) return iface.GetGenericArguments()[0];
if (iface.GetGenericTypeDefinition() == typeof(IEnumerable<>)) return iface.GetGenericArguments()[0];
}
return typeof(object);
}
}
private class WorkerProperty
{
public string PropertyName;
public IList List;
public Type ListPropertyType;
public bool IsArray;
public PropertyInfo ArrayPropertyInfo;
public override string ToString() => PropertyName;
}
}