Creating Lambda Expression for Generic Objects

Question

I need to create Expression<Func<TModel, TValue>> from my model.

How to access the property in the model:

ViewModel.CustomFieldCollection[1].PrimaryFields[3].Value

where 1 & 3 are the indexes which will updated at runtime.

I am trying to creating an expression, to be passed to a HtmlHelper, to generate an HtmlString for me.

var viewModelExpParam = Expression.Parameter(typeof(ViewModel));

var fieldParam = Expression.Property(viewModelExpParam, "CustomFieldCollection[1]");

var expression = Expression.Lambda<Func<TModel, TValue>>(fieldParam, viewModelExpParam);

But the above code gives the error while creating fieldParam, as it is not the object but a collection object.

Can i generate an expression to access ViewModel.CustomFieldCollection[1].PrimaryFields[3].Value in HtmlHelper at runtime?

Maksim Simkin · Accepted Answer · 2017-02-07 09:45:20Z

2

You could acces indexxed member via Item property (it's just a sample code, i haven't tried it out, you haven't provided any code for it :)):

var customFieldCollection = Expression.Property(viewModelExpParam,"CustomFieldCollection");
var fieldParam = Expression.Property(customFieldCollection , "Item", 
                         new Expression[] { Expression.Constant(1) });

And than:

var primaryFields = Expression.Property(fieldParam,"PrimaryFields");
var primaryFieldItem = Expression.Property(primaryFields , "Item", 
                             new Expression[] { Expression.Constant(3) });
var value = Expression.Property(primaryFieldItem, "Value");

var expression = Expression.Lambda<Func<TModel, TValue>>(value,  viewModelExpParam);

answered Feb 7, 2017 at 9:45

Maksim Simkin

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7 Comments

Lokesh Over a year ago

got a probblem: Value is referenced from ViewModel as ViewModel.Value and not from ViewModel.CustomFieldCollection[1].PrimaryFields[3].Value

Maksim Simkin Over a year ago

So you don't need all that stuff and can just call it as var value = Expression.Property(viewModelExpParam , "Value") ? And what is a problem?

Lokesh Over a year ago

sry, but i mean that as per above code my Input control is created with ID = Value, instead it should be created as ID = CustomFieldCollection[1].PrimaryFields[3].Value.

Maksim Simkin Over a year ago

what are CustomFieldCollection and PrimaryFields ? Arrays? Lists? Dictionaries?

Lokesh Over a year ago

it would be difficult to showcase that. But thanks for pointing in correct direction. The expression is generated with the hierarchy, i believe the problem is with the control generation. I will mark it as answer as it does the purpose. Thanks! :)

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