Properties marked with `[FromHeader]` attribute do not use default values when a header is not present

[pranavkm]

Consider:

public class ModelBoundType
{
     [FromHeader("Test-Header")]
     public string TestHeader { get; set;} = "Default-Value";

     public string TestModelBoundValue { get; set; } = "Default Value";

TestModelBoundValue picks up the default value when a value for the property is not found. TestHeader stays null when the a header is absent.

[yishaigalatzer]

How does this work for other model binders when the value does not exist?

[rynowak]

How does this work for other model binders when the value does not exist?

Other model binders aren't greedy and don't return true to halt model binding. The model binding system sees null and success = true and so sets the property to null.

[yishaigalatzer]

The suggestion is to fix this by changing the context to have another flag that indicates if a value was produced. This flag will be set to true by default if the value setter was invoked.

[yishaigalatzer]

We also want to consider as part of this change moving default value setting to a single location (where now it is spread between MutableObjectModelBinder and ActionExecutor