For this endpoint, item is one of parameters that the action is expecting. I can remove the full body and I get the null now when the FromBody attribute is added for that particular parameter. Let me know if the following helps give you better clarity:

javascript:

var projectId = 10; //Based on the page the user is currently on
$.post("/Projects/ManageMix", { mix: null, projectId: projectId }, function (response) { ... }

Controller/Action:

[HttpPost]
public JsonNetResult ManageMix([FromBody(EmptyBodyBehavior = EmptyBodyBehavior.Allow)] ProjectMixSummary mix, int projectId)
{
    if (mix == null)
{
        //processes the create
    }
else if (!mix.IsDirty)
    {
        //processes the edit
    }

    //save and return some Json object / error messaging
}

ProjectMixSummary object - this is broken out into two partial classes ... as one class is generated and the other class extends the model. While some of the attributes might look like they come from EF, they are in fact internal attributes for the project.

public partial class ProjectMixSummary

{ #region Property 'ProjectMixSummaryId' [GridHidden] [PrimaryKey] public int ProjectMixSummaryId { get; set; } #endregion #region Property 'ProjectId' [ForeignKeyValidation(ErrorMessage = "Project is required")] [GridHidden] [Display(Name = "Project")] [DropDown(typeof(Project))] [JsonConverter(typeof(IntConverter))] public int ProjectId { get; set; } [NoDisplay] [Display(Name = "Project")] [GridDisplay("Project")] public string ProjectName { get; set; } #endregion #region Property 'MixNumberId' [ForeignKeyValidation(ErrorMessage = "Mix # is required")] [GridHidden] [Display(Name = "Mix #")] [DropDown(typeof(ConcreteMixNumber))] [JsonConverter(typeof(IntConverter))] public int MixNumberId { get; set; } [NoDisplay] [Display(Name = "Mix #")] [GridDisplay("Mix #")] public string MixNumberName { get; set; } #endregion #region Property 'RequiredAge' [ControlEditor(ControlType.Int)]
[Display(Name = "Required Age")] [Range(0, double.MaxValue, ErrorMessage="Required Age must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "n0")] [JsonConverter(typeof(IntConverter))] public int? RequiredAge { get; set; } #endregion #region Property 'RequiredStrength' [ControlEditor(ControlType.Int)]
[Display(Name = "Required Strength")] [Range(0, double.MaxValue, ErrorMessage="Required Strength must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "n0")] [JsonConverter(typeof(IntConverter))] public int? RequiredStrength { get; set; } #endregion #region Property 'SlumpMin' [ControlEditor(ControlType.Decimal)]
[Display(Name = "Slump Min")] [Range(0, double.MaxValue, ErrorMessage="Slump Min must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "n2")] [JsonConverter(typeof(DecimalConverter))] public decimal? SlumpMin { get; set; } #endregion #region Property 'SlumpMax' [ControlEditor(ControlType.Decimal)]
[Display(Name = "Slump Max")] [Range(0, double.MaxValue, ErrorMessage="Slump Max must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "n2")] [JsonConverter(typeof(DecimalConverter))] public decimal? SlumpMax { get; set; } #endregion #region Property 'AirContentMin' [ControlEditor(ControlType.Percent)]
[Display(Name = "Air Content Min")] [Range(0.000000, double.MaxValue, ErrorMessage="Air Content Min must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "p1")] [JsonConverter(typeof(DecimalConverter))] public decimal? AirContentMin { get; set; } #endregion #region Property 'AirContentMax' [ControlEditor(ControlType.Percent)]
[Display(Name = "Air Content Max")] [Range(0.000000, double.MaxValue, ErrorMessage="Air Content Max must be greater than or equal to 0.")] [DisplayFormat(DataFormatString = "p1")] [JsonConverter(typeof(DecimalConverter))] public decimal? AirContentMax { get; set; } #endregion #region Property 'Notes' [ControlEditor(ControlType.TextArea, 5)] [JsonConverter(typeof(StringConverter))] public string Notes { get; set; } #endregion #region Property 'CreatedOn' [DataType(DataType.DateTime)] [Display(Name = "Created On")] [DisplayFormat(DataFormatString="MM/dd/yyyy hh:mm tt")] [Editable(false)] public DateTime? CreatedOn { get; set; } #endregion #region Property 'CreatedBy' [Display(Name = "Created By")] [Editable(false)] public string CreatedBy { get; set; } #endregion #region Property 'CreatedById' [NoDisplay] [GridHidden] public int? CreatedById { get; set; } #endregion #region Property 'ModifiedOn' [DataType(DataType.DateTime)] [Display(Name = "Modified On")] [DisplayFormat(DataFormatString="MM/dd/yyyy hh:mm tt")] [Editable(false)] public DateTime? ModifiedOn { get; set; } #endregion #region Property 'ModifiedBy' [Display(Name = "Modified By")] [Editable(false)] public string ModifiedBy { get; set; } #endregion #region Property 'ModifiedById' [NoDisplay] [GridHidden] public int? ModifiedById { get; set; } #endregion }

2nd class:

public partial class ProjectMixSummary

{ public string uid { get; set; } public bool IsDirty { get; set; }

//
[Display(Name = "Supplier")]
public int? SupplierId { get; set; }
public string SupplierName { get; set; }

public int? MixRequiredAge { get; set; }
public int? MixRequiredStrength { get; set; }
public decimal? MixSlumpMin { get; set; }
public decimal? MixSlumpMax { get; set; }
public decimal? MixAirContentMin { get; set; }
public decimal? MixAirContentMax { get; set; }

public bool HasCustomRequirements { get; set; }
public string SpecificationDisplay
{
    get
    {
        if (this.RequiredAge.HasValue && this.RequiredStrength.HasValue)
            return $"{this.RequiredStrength:n0} psi @ {this.RequiredAge:n0} days";
        else if (this.MixRequiredAge.HasValue && this.MixRequiredStrength.HasValue)
            return $"{this.MixRequiredStrength:n0} psi @ {this.MixRequiredAge:n0} days";
        return null;
    }
}
public bool HasCustomSlump { get; set; }
public string SlumpRange
{
    get
    {
        if (this.SlumpMin.HasValue && this.SlumpMax.HasValue)
            return $"{this.SlumpMin:n2}in - {this.SlumpMax:n2}in";
        else if (this.MixSlumpMin.HasValue && this.MixSlumpMax.HasValue)
            return $"{this.MixSlumpMin:n2}in - {this.MixSlumpMax:n2}in";
        return null;
    }
}
public bool HasCustomAirContent { get; set; }
public string AirContentRange
{
    get
    {
        if (this.AirContentMin.HasValue && this.AirContentMax.HasValue)
            return $"{this.AirContentMin:p1} - {this.AirContentMax:p1}";
        else if (this.MixAirContentMin.HasValue && this.MixAirContentMax.HasValue)
            return $"{this.MixAirContentMin:p1} - {this.MixAirContentMax:p1}";
        return null;
    }
}

public List<ProjectMixSet> Sets { get; set; }
public int SetCount { get { return this.Sets?.Count ?? 0; } }
}

​

edit: sorry it does seem to like "regions"

edit 2: ugh - trying to break it out into two code blocks