argumentexception

System.ArgumentException: An item with the same key has already been added

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Problem Dictionaries require keys to be unique. When you try to add a key/value to a dictionary and the key already exists, you get the following exception: System.ArgumentException: An item with the same key has already been added. This can happen when you use Dictionary.Add() or when you initialize a dictionary using the “curly brace” … Read more

CA2208: Instantiate argument exceptions correctly

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The CA2208 code analysis rule checks for common mistakes when constructing argument exceptions. There are three main argument exception classes: ArgumentException, ArgumentNullException, and ArgumentOutOfRangeException. Unfortunately, it’s easy to make a mistake when using these. I’ll explain the common mistakes that CA2208 checks for and how to fix them (and when to suppress the warning instead). … Read more

C# – Using reflection to get properties

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You can get a list of a type’s properties using reflection, like this: Note: If you have an object, use movie.GetType().GetProperties() instead. This outputs the following: When you use GetProperties(), it returns a list of PropertyInfo objects. This gives you access the property’s definition (name, type, etc…) and allows you to get and modify its … Read more

Complex DataBinding accepts as a data source either an IList or an IListSource

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If you try to set a list control’s DataSource to a type it can’t handle, then you’ll get the following exception: System.ArgumentException: Complex DataBinding accepts as a data source either an IList or an IListSource. (Parameter ‘value’)at System.Windows.Forms.ListControl.set_DataSource(Object value) Note: This applies to all controls that subclass ListControl, such as ComboBox and ListBox. This is … Read more

InvalidOperationException: Unable to resolve service for type

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When the built-in dependency injection functionality is trying to create a type, it tries to resolve all of the constructor parameters. If it can’t resolve one of the parameters, it’ll throw a variation of one of these exceptions: The error you get will depend on how you’re doing registration. First, I’ll show the solution. Then … Read more

Collection was modified; enumeration operation may not execute

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If you try to add/remove items from a collection while it’s being looped over in a foreach loop (enumerated), then you’ll get the following exception: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.at System.Collections.Generic.List`1.Enumerator.MoveNext() This error can happen in two scenarios: The solution to this problem depends on which scenario you’re in. In this … Read more

C# – Read a custom config section from app.config

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In this this article, I’ll show the simplest to get a custom config section from app.config and load it into your own config class. You can implement IConfigurationSectionHandler and use ConfigurationManager.GetSection() to do this. I’ll show the steps below. 1 – Add a custom config class The first step is to create a class that’ll … Read more