Added common errors page

docs/errata/README.md

@@ -6,4 +6,6 @@ If you find any mistakes in the seventh edition, *C# 11 and .NET 7 - Modern Cros
 
 [**Improvements** (26 items)](improvements.md): Changes to text or code that would improve the content. These are optional.
 
+[**Common Errors** (3 items)](common-errors.md): These are some of the most common errors that a reader might encounter when trying to get code in book tasks to work, or when trying to write your own code. 
+
 All errata and improvements will be included in the 8th edition planned for publishing in November 2023. To be included they must be submitted by mid-September 2023.

docs/errata/common-errors.md

@@ -0,0 +1,119 @@
+**Common Errors and How to Fix Them** (3 items)
+
+If you have suggestions for other common errors, then please [raise an issue in this repository](https://github.com/markjprice/cs12dotnet8/issues) or email me at markjprice (at) gmail.com.
+
+- [Missing types and members in a utlity class](#missing-types-and-members-in-a-utlity-class)
+  - [CS0103 The name 'DoSomething' does not exist in the current context](#cs0103-the-name-dosomething-does-not-exist-in-the-current-context)
+  - [CS0122 'Util.DoSomething()' is inaccessible due to its protection level](#cs0122-utildosomething-is-inaccessible-due-to-its-protection-level)
+- [Missing types and members in the Program class](#missing-types-and-members-in-the-program-class)
+- [Microsoft introduces a bug in a later version](#microsoft-introduces-a-bug-in-a-later-version)
+
+# Missing types and members in a utlity class
+
+## CS0103 The name 'DoSomething' does not exist in the current context
+
+You might create a utility class with a method, as shown in the following code:
+```cs
+namespace Common.Errors;
+
+public class Util
+{
+  static void DoSomething()
+  {
+    Console.WriteLine("Doing something.");
+  }
+}
+```
+
+You try to call the method directly (without any class name prefix) in the `Program.cs` file, as shown in the following code:
+```cs
+using Common.Errors;
+
+DoSomething(); // This causes: CS0103 The name 'DoSomething' does not exist in the current context.
+```
+
+The class is declared inside the `Common.Errors` namespace and you have imported the `Common.Errors` namespace in `Program.cs` so why can't it be found? 
+
+First, you need to *statically import the class* rather than just import it's namespace, as shown in the following code:
+```cs
+using static Common.Errors.Util; // Statically import the class to call its static methods.
+```
+
+## CS0122 'Util.DoSomething()' is inaccessible due to its protection level
+
+But now you get a different compiler error, as shown in the following code:
+```cs
+DoSomething(); // This causes: CS0122 'Util.DoSomething()' is inaccessible due to its protection level.
+```
+
+Next, by default members of a class are `private` so you must explicitly make the method `public` or `internal`, as shown in the following code:
+```cs
+namespace Common.Errors;
+
+public class Util
+{
+  public static void DoSomething()
+  {
+    Console.WriteLine("Doing something.");
+  }
+}
+```
+
+Now the method call works.
+
+# Missing types and members in the Program class
+
+In my book, I show how to define functions that can be easily called directly in the `Program.cs` file because you add them to the `partial Program` class definition. You can put these functions in a separate file, for example, `Program.Helpers.cs`, as shown in the following code:
+
+> Note: The filename is not important and you can have as many files as you like, for example, `Program.Helpers.cs`, `Program.Functions.cs`, `Any.Thing.cs`, and even `Muppets.cs`.
+
+```cs
+// A file named Program.Helpers.cs.
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Common.Errors
+{
+  partial class Program
+  {
+    static void DoSomethingElse()
+    {
+      Console.WriteLine("Doing something else.");
+    }
+  }
+}
+```
+
+> Note: The above file was created by the Visual Studio 2022 project item template. It imports five commonly used namespaces and wraps the class in a namespace that uses the name of the project.
+
+You try to call the method directly (without any class name prefix) in the `Program.cs` file, as shown in the following code:
+```cs
+DoSomethingElse(); // This causes: CS0103 The name 'DoSomething' does not exist in the current context.
+```
+
+The class is declared as `Program` and uses the `partial` keyword so why doesn't it merge with the auto-generated `Program` class? Because it is a different namespace and therefore you now have two classes named `Program`. The fully qualified name of your class is `Common.Errors.Program`. The fully-qualified name of the auto-generated class is just `Program`.
+
+To fix the problem, delete the explicit namespace (and remove the unncessary imported namespaces too), as shown in the following code:
+```cs
+// A file named Program.Helpers.cs.
+partial class Program
+{
+  static void DoSomethingElse()
+  {
+    Console.WriteLine("Doing something else.");
+  }
+}
+```
+
+# Microsoft introduces a bug in a later version
+
+Although rare, it is possible that by using a later version of a NuGet package than the one I used to write the book, you experience different behavior, especially negative behavior if it is due to a bug. 
+
+For example, in the `Microsoft.Extensions.Configuration.Binder` package, version `7.0.3` has a bug. Previous versions from `7.0.0` to `7.0.0` do not have the bug.
+
+You can read more here: https://github.com/markjprice/cs11dotnet7/blob/main/docs/errata/errata.md#page-178---reviewing-project-packages
+
+If you add packages using the Visual Studio 2022 user interface or the `dotnet add package` command-line tool then it will use the most recent version. If you have problems, try manually reverting to an older version.