How can I get the executing assembly version?

Two options... regardless of application type you can always invoke:

Assembly.GetExecutingAssembly().GetName().Version

If a Windows Forms application, you can always access via application if looking specifically for product version.

Application.ProductVersion

Using GetExecutingAssembly for an assembly reference is not always an option. As such, I personally find it useful to create a static helper class in projects where I may need to reference the underlying assembly or assembly version:

// A sample assembly reference class that would exist in the `Core` project.
public static class CoreAssembly
{
    public static readonly Assembly Reference = typeof(CoreAssembly).Assembly;
    public static readonly Version Version = Reference.GetName().Version;
}

Then I can cleanly reference CoreAssembly.Version in my code as required.

In MSDN, Assembly.GetExecutingAssembly Method, is remark about method "getexecutingassembly", that for performance reasons, you should call this method only when you do not know at design time what assembly is currently executing.

The recommended way to retrieve an Assembly object that represents the current assembly is to use the Type.Assembly property of a type found in the assembly.

The following example illustrates:

using System;
using System.Reflection;

public class Example
{
    public static void Main()
    {
        Console.WriteLine("The version of the currently executing assembly is: {0}",
                          typeof(Example).Assembly.GetName().Version);
    }
}

/* This example produces output similar to the following:
   The version of the currently executing assembly is: 1.1.0.0

Of course this is very similar to the answer with helper class "public static class CoreAssembly", but, if you know at least one type of executing assembly, it isn't mandatory to create a helper class, and it saves your time.

using System.Reflection;
{
    string version = Assembly.GetEntryAssembly().GetName().Version.ToString();
}

Remarks from MSDN http://msdn.microsoft.com/en-us/library/system.reflection.assembly.getentryassembly%28v=vs.110%29.aspx:

The GetEntryAssembly method can return null when a managed assembly has been loaded from an unmanaged application. For example, if an unmanaged application creates an instance of a COM component written in C#, a call to the GetEntryAssembly method from the C# component returns null, because the entry point for the process was unmanaged code rather than a managed assembly.