This error occurs when you are typically trying to present an object as a string. The solution here is to append ".ToString()," that now represents the object as a string.

Again this has to do with improperly trying to convert a value. To remedy this properly cast or box the value in question.

This runtime error is more often than not caused by your code trying to reference an object that is not available, and for the most part you've probably closed the object or set it to null or nothing somewhere in your code in all valiant attempts to clean up any unnecessary objects from memory. For example, a connection object you've opened and used, but now somewhere you've closed it and set it to nothing, and somewhere after in the code it's being called again and when it does you'll get this error. Find and delete the line and close and null your object later on. Notwithstanding, this runtime error will also occur in the circumstances shown above in CS0119.

This error is produced by a number of sources.

1. If you call an object that requires parentheses () after the command, you'll get this error. Ex. database.Open() not database.Open
2. Using an assignment operator as opposes to an equality operator
   

This happens when you are referencing a web form control or variable that either doesn't exist or is named different. Also by chance the control in question may be missing the mandatory runat="server" option.

This can happen from any number of reasons in your code. Some common ones are:

1. Not setting the object's proper data type
2. An expression that binds data was incorrectly parsed against the object at runtime In Datagrids, for instance, this occurs when omitting the DataBinder.Eval method on a data field

This can happen usually when you forget to use the new keyword when instantiating an object or invoking a constructor.