Accessing non-static member in c# Lazy without using default constructor

Question

I want to add a Lazy to my class. To access non-static members inside the initialization method of the Lazy, the best-practice method (see https://stackoverflow.com/a/14029443/10406502) is to define the Lazy inside the default constructor. This is not possible in my project, because it's a partial class and the default constructor is already defined by some auto-generated code (Entity Framework).

Is there a way to use Lazy accessing non-static class members without implementing it in the default constructor?

private Lazy<int> lazyGetSum = new Lazy<int>(new Func<int>(() => X + Y)); — Robert Harvey
– Robert Harvey, Commented Oct 5, 2021 at 14:34
@RobertHarvey Doesn't work, because you can't access non-static members inside Func<int> (in your example). — André Reichelt
– André Reichelt, Commented Oct 5, 2021 at 14:38
@RobertHarvey See the question linked in the OP. That does e.g. new Lazy<int>(() => X + Y) where X and Y are instance members. — canton7
– canton7, Commented Oct 5, 2021 at 14:39
You'll have to do a bit of poking around on the Internet, but I'm pretty sure this is the right approach. See also entityframework.net/knowledge-base/31793997 — Robert Harvey
– Robert Harvey, Commented Oct 5, 2021 at 14:50

André Reichelt · Accepted Answer · 2021-10-06 06:01:22Z

Thanks to Robert Harvey, I found the following solution: First, you have to add some stuff to your data model template MyModeName.tt, which you can find under the MyModelName.edmx in your project. I added a new partial method private partial void OnConstructorCompletion(); and called it at the end of the constructor. I modified the document as follows (lines 27-67):

<#=codeStringGenerator.EntityClassOpening(entity)#>
{
    private partial void OnConstructorCompletion();

<#
    var propertiesWithDefaultValues = typeMapper.GetPropertiesWithDefaultValues(entity);
    var collectionNavigationProperties = typeMapper.GetCollectionNavigationProperties(entity);
    var complexProperties = typeMapper.GetComplexProperties(entity);

    if (propertiesWithDefaultValues.Any() || collectionNavigationProperties.Any() || complexProperties.Any())
    {
#>
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public <#=code.Escape(entity)#>()
    {
<#
        foreach (var edmProperty in propertiesWithDefaultValues)
        {
#>
        this.<#=code.Escape(edmProperty)#> = <#=typeMapper.CreateLiteral(edmProperty.DefaultValue)#>;
<#
        }

        foreach (var navigationProperty in collectionNavigationProperties)
        {
#>
        this.<#=code.Escape(navigationProperty)#> = new HashSet<<#=typeMapper.GetTypeName(navigationProperty.ToEndMember.GetEntityType())#>>();
<#
        }

        foreach (var complexProperty in complexProperties)
        {
#>
        this.<#=code.Escape(complexProperty)#> = new <#=typeMapper.GetTypeName(complexProperty.TypeUsage)#>();
<#
        }
#>

        OnConstructorCompletion();
    }

Next, I implemented the method where required inside of my entity class:

public partial class MyEntity
{
    public Lazy<ICollection<MyNavigationEntity>> NavigationProperty;

    private partial void OnConstructorCompletion()
    {
        NavigationProperty= new Lazy<ICollection<MyNavigationEntity>>(() =>
        {
            DatabaseEntities db = (DatabaseEntities)EntityContextExtensions.GetDbContextFromEntity(this);

            return db.MyNavigationEntity
                .Where(ne => ne.ID == this.ID_MyNavigationEntity)
                .ToHashSet();
        });
    }
}

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