Part 1: Develop on sanbox solution and resolve error

Step 1: Open Visual Studio 2010 | Add New Project | Sharepoint | Empty Sharepoint Project and named is DevSanbox

Step 2: Choose your Web App and choose deploy sanbox

Step 3: Add new item | Web Part and named is DevSanbox

Step 4: code in event  CreateChildControls as

Code here

protected override void CreateChildControls()

        {

            Literal literalResult = new Literal();

            this.Controls.Add(literalResult);

            SPSite spSite = SPControl.GetContextSite(Context);

            SPWeb spWeb = spSite.OpenWeb();           

            SPList spList = spWeb.Lists["Tasks"];

            SPQuery spQuery=new SPQuery();

            SPListItemCollection spListItemCollection = spList.GetItems(spQuery);

            foreach (SPListItem item in spListItemCollection)

            {

                literalResult.Text += item["Title"] + "||";

            }

        }

Step 5: Build project and Package it

Step 6: Open your web application =>  back to top level site, Click Site Actions | Site Setting | Click “Solutions” link | Upload Solution

Step 7: Choose  your .WSP in bin folder and Active it

Step 8: Result as

Step 9: Add Web Part as

Step 10: Error appear

Step 11: Solve problem. Replace code as

protected override void CreateChildControls()

        {

            Literal literalResult = new Literal();

            this.Controls.Add(literalResult);

            //SPSite spSite = SPControl.GetContextSite(Context);

            //SPWeb spWeb = spSite.OpenWeb();           

            using (SPWeb spWeb = SPContext.Current.Web)//Important

            {

                SPList spList = spWeb.Lists["Tasks"];

                SPQuery spQuery = new SPQuery();

                SPListItemCollection spListItemCollection = spList.GetItems(spQuery);

                foreach (SPListItem item in spListItemCollection)

                {

                    literalResult.Text += item["Title"] + "||";

                }

            }

           

        }

Step  12: Deactive Solution

Step 13: Upload solution again and Active It

Step 14: Result as