SP.UI.$create_DialogOptions is not a function issue in visual webpart

When we try to invoke the SharePoint modal dialog from visual webpart, sometimes we will get an error as "SP.UI.$create_DialogOptions is not a function". In order to resolve the issue, explicitly call LoadSodByKey function from document ready method of visual webpart.

$(document).ready(function () {
LoadSodByKey("sp.js", function () {});
InvokeSPDialog();
});

SharePoint modal dialog:

function InvokeSPDialog() {
    var diaOptions = SP.UI.$create_DialogOptions();
    diaOptions.url = 'http://siteurl/_layouts/15/Test.aspx',
    diaOptions.title = "Test";
    diaOptions.width = 300;
    diaOptions.height = 100;
    diaOptions.dialogReturnValueCallback = Function.createDelegate(null, CloseCallBack);
    SP.UI.ModalDialog.showModalDialog(diaOptions);
}
function CloseCallBack(result, returnValue) {
        alert('ok');  

}

Hope this resolves the issue!