Detect init-only property using reflection

ReflectionExtensions.cs

public static class ReflectionExtensions
{
    /// <summary>
    /// Gets a value indicating whether property has init accessor declared.
    /// </summary>
    /// <param name="propertyInfo"></param>
    /// <returns>Returns <c>true</c> if the property is declared as init-only; otherwise returns <c>false</c>.</returns>
    public static bool IsInitOnly(this PropertyInfo propertyInfo)
    {
        if (propertyInfo == null)
            throw new ArgumentNullException(nameof(propertyInfo));

        // return type of init-only property setter has custom modifier applied to it: modreq(System.Runtime.CompilerServices.IsExternalInit) 
        // e.g.: .set instance void modreq(System.Runtime.CompilerServices.IsExternalInit) Net5FeaturesDemo.TestClass::set_TestProperty(int32)

        return propertyInfo
            .SetMethod?
            .ReturnParameter
            .GetRequiredCustomModifiers()
            .Any(modifierType => modifierType.FullName == "System.Runtime.CompilerServices.IsExternalInit") == true;        
            // .Contains(typeof(System.Runtime.CompilerServices.IsExternalInit)) == true; // type is only available in .NET 5
    }
}