Customize the Error Message in ASP.NET MVC

Add the following keys into Application_Start() at Global.asax

ClientDataTypeModelValidatorProvider.ResourceClassKey = "YourResourceName";
DefaultModelBinder.ResourceClassKey = "YourResourceName";

Create YourResourceName.resx inside App_GlobalResources folder and add the following keys
FieldMustBeDate The field {0} must be a date.
FieldMustBeNumeric The field {0} must be a number.
PropertyValueInvalid The value '{0}' is not valid for {1}.
PropertyValueRequired A value is required.

Convert XML to C# Object

https://www.codeproject.com/Articles/1163664/Convert-XML-to-Csharp-Object

Get a property value based on the name

Add to any Class:

public class Foo
{
    public object this[string propertyName]
    {
        get { return this.GetType().GetProperty(propertyName).GetValue(this, null); }
        set { this.GetType().GetProperty(propertyName).SetValue(this, value, null); }
    }

    public string Bar { get; set; }
}

Then, you can use as:

Foo f = new Foo();
// Set
f["Bar"] = "asdf";
// Get
string s = (string)f["Bar"];

Error occurred in deployment step 'Activate Features': Operation is not valid due to the current state of the object

1. Retract solution
2. End task "vssphost5.exe" in Task Manager
3. Restart SharePoint Timer Service
4. Deploy solution

How to make executeQueryAsync() behave synchronously?

You can use JavaScript call backs or Promises/deferreds to work with JavaScript client object model

CallBack Example:

$(document).ready(function () {
        //don't exectute any jsom until sp.js file has loaded.        
        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', prepareTables);
    });

function prepareTables() {
    getItemsWithCaml('External User Account Request',
            function (camlItems) {
                var listItemEnumerator = camlItems.getEnumerator();
                while (listItemEnumerator.moveNext()) {
                    var listItem = listItemEnumerator.get_current();
                    console.log(listItem.get_item('Title'));
                } console.log('Completed table prparation.');
            },
            function (sender, args) {
                console.log('An error occured while retrieving list items:' + args.get_message());
            });
}    

function getItemsWithCaml(listTitle, success, error) {
    var clientContext = new SP.ClientContext.get_current();
    var list = clientContext.get_web().get_lists().getByTitle(listTitle);
    var camlQuery = new SP.CamlQuery();
    var camlItems = list.getItems(camlQuery);
    clientContext.load(camlItems);
    clientContext.executeQueryAsync(
            function () {
                success(camlItems);
            },
            error
        );
};

Promises Example:

$(document).ready(function () {
        //don't exectute any jsom until sp.js file has loaded.        
        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', prepareTables);
    });    

function prepareTables() {
    getItemsWithCaml('External User Account Request').then(
            function (camlItems) {
                var listItemEnumerator = camlItems.getEnumerator();
                while (listItemEnumerator.moveNext()) {
                    var listItem = listItemEnumerator.get_current();
                    console.log(listItem.get_item('Title'));
                } console.log('Completed table prparation.');
            },
            function (sender, args) {
                console.log('An error occured while retrieving list items:' + args.get_message());
            }
        );   
}    

function getItemsWithCaml(listTitle) {
    //use of $.Deferred in the executeQueryAsync delegate allows the consumer of this method to write 'syncronous like' code
    var deferred = $.Deferred();
    var clientContext = new SP.ClientContext.get_current();
    var list = clientContext.get_web().get_lists().getByTitle(listTitle);
    var camlQuery = new SP.CamlQuery();        
    var items = list.getItems(camlQuery);
    clientContext.load(items);
    clientContext.executeQueryAsync(
        Function.createDelegate(this,
            function () { deferred.resolve(items); }),
        Function.createDelegate(this,
            function (sender, args) { deferred.reject(sender, args); }));

    return deferred.promise();
};
Origin: http://www.sharepointnadeem.com/2014/10/sharepoint-using-deferredspromises-or.html