String property is generated a nullable with no obvious way to mark it non-nullable

[R2D221]

Describe the bug

I'm trying to declare a property for a custom YearMonthPicker. I'm adding a string property, and I'm expecting it not to be null because I'll be providing a default value. However, even tho the property itself is not marked as nullable, the generated code is:

    /// <summary>
    /// Default value: "MMMM yyyy"
    /// </summary>
    [global::System.CodeDom.Compiler.GeneratedCode("DependencyPropertyGenerator", "1.4.0.0")]
    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
    public string? StringFormat
    {
        get => (string?)GetValue(StringFormatProperty);
        set => SetValue(StringFormatProperty, value);
    }

Steps to reproduce the bug

Declare a class, and add the following attribute:

[DependencyProperty<string>("StringFormat", DefaultValue = "MMMM yyyy")]

Expected behavior

    /// <summary>
    /// Default value: "MMMM yyyy"
    /// </summary>
    [global::System.CodeDom.Compiler.GeneratedCode("DependencyPropertyGenerator", "1.4.0.0")]
    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
    public string StringFormat
    {
        get => (string)GetValue(StringFormatProperty);
        set => SetValue(StringFormatProperty, value);
    }

Screenshots

No response

NuGet package version

1.4.0

IDE

Visual Studio 2022

Additional context

No response

[HavenDV]

Yes, in general this sounds logical.
But there are two ways here - make a Nullable parameter for the attribute with the default value true and the ability to make it false
Or make the property non-nullable automatically if DefaultValue is set. There may be problems here if under certain conditions this property may be null sometimes.

[R2D221]

I was thinking that, to guarantee a non-null value, a coerce-value callback should also be provided. Maybe having a NonNullable property could make the DefaultValue and Coerce method required, with the correct nullability for the method...

[HavenDV]

Added in latest commit: 407759e

[BryceBarbara]

I'm unable to get this to work as I always get the following errors:

• Partial method 'X.CoerceY(SomeType?)' must have an implementation part because it has accessibility modifiers.
• Type 'X' already defines a member called 'CoerceY' with the same parameter types

Can you provide an example of how it should be used?

[HavenDV]

https://github.com/HavenDV/DependencyPropertyGenerator/blob/ba63c10edbd86bd875b1e1bdd54b8bf7d2a2fbe5/src/tests/DependencyPropertyGenerator.SnapshotTests/Tests.Properties.cs#L158C1-L179C6