Incorrect Format


Here's the scenario: I'm building a basic 3-layer Web application for a client with business and data layers. Queries are performed in stored procedures, just basic ADO.NET. The table I am attempting to insert a record into has a range a field types such as strings, ints, decimals, etc.

If the user is entering integer or decimal values via texboxes, which is usually the case, then you have to expect either no data or a string of possibly mal-formed data. You must filter out the mal-formed data with validation and/or convert the resulting input data to the expected database type.

If you have a database field called SquareFeet of type int and you attempt to insert the following in your code-behind insert method, you will probably see this error:

// Results in error message
AddSomething(int squareFeet)


You must be careful if you are trying to fetch, say, an int or decimal as input:

// Correct way if SquareFeet field is an int
int squareFeet
if (int.TryParse(squareFeetTxt.Text.Trim(), out squareFeet))
    squareFeet = Convert.ToInt32(squareFeetTxt.Text.Trim());
     squareFeet = 0;
// Now insert the value as part of the record


// Correct way if SquareFeet field is a decimal
int squareFeet
if (decimal.TryParse(squareFeetTxt.Text.Trim(), out squareFeet))
    squareFeet = Convert.ToDecimal(squareFeetTxt.Text.Trim());
     squareFeet = 0.0m;
// Now insert the value as part of the record



Refer to these links for the differences between Convert.ToInt, int.Parse and Int.TryParse:

Refer to this link for an explanation of the 0.0m decimal syntax:

Moral of the Story
If you are learning any language, your first priority must be to understand the types thoroughly. Resist the temptation to gloss over the grammar just to get to the fancy stuff! :-)

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C# 4.0 is the next version of the C# language being shipped with VS 2010. Personally, I'm still coming to terms with Generics as introduced in C# 2.0 and LINQ which came with 3.0. Anders Hejlsberg touched on the Covariance/Contravariance topic in his TechEd presentation in LA last month entitled "The Future of C#".

Evolution of C# - Anders Hejlsberg


The terms Invariance, Covariance and Contravariance are not by any means new. Anyone coming from a computer science background will have encountered the terms. Prior to attending Anders' session, I hadn't given this much conscious thought in a long time - we tend to know what we can and can't do within the syntactical constraints of a language - and tend not to question things further. However, in C# 4.0, delegates and interfaces will play nicer when working with generics. So much attention is likely to be focused on the dynamic programming additions in C# 4.0 (mainly in support of LINQ), that this addition may go unnoticed.

Variance in C# 4.0 - Anders Hejlsberg - Anders Hejlsberg



Invariant: A return parameter is invariant if we must use the exact match of the type name. In other words, neither covariance nor contravariance is permitted

Covariant: A parameter is covariant if we can use a more derived type as a substitute for the parameter type. In other words, a subclass instance can be used where a parent class instance was expected.

Contravariant: A return value is contravariant if we can assign the return type to a variable of a less derived type than the parameter. In other words, a super class instance can be used where a subclass instance was expected.

Variance in C# 4.0 - Anders Hejlsberg - Anders Hejlsberg


Generally, C# supports covariant parameters and contravariant return types. There has always been support for covariance and contravariance in C# - C# 4.0 will just ensure that generic delegates and interfaces will also behave they way we would expect.

Eric Lippert is the developer responsible for this feature of the C# 4.0 release and has an eleven-part blog series on just this topic. I would also recommend reading Charlie Calvert's article on this topic for some code samples.

Side Note: You do not need VS 2010 to experiment with these features - just download the framework to a test VM.

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.NET 4.0 | C#

You've checked your namespaces and references a hundred times but you're still getting this annoying error when accessing code files in a sub-folder?

Are you Missing an Assembly Reference?

There is more than one cause of this error, for example, you will sometimes see it when attempting to use the App_Code folder within an ASP.NET 2.0 Web Application project.

On this particular occassion, we are building an ASP.NET 2.0 Web Application project in C# using VS 2005. The folder containing the code file is just a regular folder, but the same error message appears.

Here's a fix that might work for you. Right-click the file you are trying to access in Solution Explorer.

File Properties in Solution Explorer

Select Properties which will open the Properties window. Go to the Build Action DropDown menu.

File Build Action

Select 'Compile' for the build action. In this case, it had been to be set to 'Content'. Now compile as normal and you should be able to reference the file.

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