Json.NET require all properties on deserialization

If your model has properties that your JSON may omit, and you want that to be an error, add the attribute [JsonObject(ItemRequired=Required.Always)] to your classes:

Type: Required

A value indicating whether the object’s properties are required.

Possible values for Required are:

• Default: The property is not required. The default state.
• AllowNull: The property must be defined in JSON but can be a null value.
• Always: The property must be defined in JSON and cannot be a null value.
• DisallowNull: The property is not required but it cannot be a null value [if present]. (Json.NET 8.0.1 and later.)

The setting is inherited so can be added to a generic base class.

Update

To do it globally for all objects, subclass the DefaultContractResolver and add the ItemRequired flag to all object contracts:

public class RequireObjectPropertiesContractResolver : DefaultContractResolver
{
    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        var contract = base.CreateObjectContract(objectType);
        contract.ItemRequired = Required.Always;
        return contract;
    }
}

And then later, in settings:

var settings = new JsonSerializerSettings { ContractResolver = new RequireObjectPropertiesContractResolver() };

Notes:

• If you don’t want to require JSON properties when your f# member is optional see this answer to this question and also the question Json.NET make property required based on property type.

• This contract resolver applies a default setting of Required.Always to all object properties, but will not override JsonProperty.AttributeRequired when applied directly. If you need that, see e.g. How to override the “Required.Always” in newtonsoft json.

• As stated in the question, the setting MissingMemberHandling = MissingMemberHandling.Error solves a complimentary problem: if your JSON may have properties that your model omits, and you want that to be an error, use MissingMemberHandling.Error. See: MissingMemberHandling setting.

• You may want to cache the contract resolver for best performance.