check if a generic property is null using equals method

Question

User254389092 posted

In my code, 

public static IHtmlString RecipientSelectorFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string buttonText, bool isReadOnly, string onClickFunctionName)
{

...

output.AppendFormat(@"<div class='input-group '>
<input id='{0}' name='{0}' type='text' class='input-sm form-control{7}' value='{6}' title='{6}' />
<div class='input-group-btn'>
<button type='button' class='btn btn-default dropdown-toggle' {5} title='{1}' onclick='{3}(""{2}"",""{4}"")'>{1}
<span class='caret'></span>
</button>
</div>
</div>",
name,
buttonTitle,
recipientSelectorTitle,
buttonFunction,
urlHelper.Action("_RecipientSelector", "Recipient", new { Id = name, isPrivate = false }),
isReadOnly ? "disabled='disabled'" : "",
property == null ? string.Empty : property.ToString(),htmlHelper.ViewData.ModelState[name] != null && htmlHelper.ViewData.ModelState[name].Errors.Any() ?
" input-validation-error" : string.Empty);

}

My code factor tool suggests me to "Use a comparison to 'default(TProperty)' instead or add a constraint to 'TProperty' so that it can't be a value type." I have tried:

output.AppendFormat(@"<div class='input-group '>
<input id='{0}' name='{0}' type='text' class='input-sm form-control{7}' value='{6}' title='{6}' />
<div class='input-group-btn'>
<button type='button' class='btn btn-default dropdown-toggle' {5} title='{1}' onclick='{3}(""{2}"",""{4}"")'>{1}
<span class='caret'></span>
</button>
</div>
</div>",
name,
buttonTitle,
recipientSelectorTitle,
buttonFunction,
urlHelper.Action("_RecipientSelector", "Recipient", new { Id = name, isPrivate = false }),
isReadOnly ? "disabled='disabled'" : "",
property.Equals(null, default(TProperty)) ? string.Empty : property.ToString(),
htmlHelper.ViewData.ModelState[name] != null && htmlHelper.ViewData.ModelState[name].Errors.Any() ?
" input-validation-error" : string.Empty);

This code returns compile time error:

member object.Equals(object, object) can not be accessed with an instance reference; Qualify it with a type name instead. 

How can I check this expression if it has a null value inside that element? Please help me with a fix

Luc

 

Answer 1

User303363814 posted

property.Equals( default(TProperty))

But it would probably better to tell the compiler about any extra information you have about TProperty with a constraint.  What is the range of types that can be passed?

Answer 2

User254389092 posted

Hi PaulTheSmith,

I need to convert below line of code:

property == null ? string.Empty : property.ToString()

to use Equals to compare. Can it be done?

Luc

Answer 3

User-719153870 posted

Hi Lucifer_deep,

property.Equals(null, default(TProperty)) ? string.Empty : property.ToString()

What is this property? Is this an instance of Generics or a common object like string?

If you are trying to use the Equals(Object) method, you will only need to set the Object to null, like below:

string property = "abc";
Response.Write(property.Equals(null) ? string.Empty : property.ToString());

As for the error member object.Equals(object, object) can not be accessed with an instance reference; Qualify it with a type name instead, you want to check this thread.

Best Regard,

Yang Shen

Answer 4

User254389092 posted

I think I need to use the code from your link. But I did not quite understand the code. Can you give me a code that works in my case?

Unfortunately, I did not get a proper understanding of the issue.

Answer 5

User303363814 posted

Remove 

property == null

and replace with

property.Equals(default(TProperty))

Or, equivalently, delete 

null,

from the code you showed which had a compilation error

Answer 6

User254389092 posted

Thanks PaulTheSmith,

I tried that but unfortunately, this seems to be another code smell than a valid solution. I am still working on it. But right now, I do not have a valid solution to it. Is there another way to achieve this?

Luc

Answer 7

User-719153870 posted

Hi Luc,

Can you provide the definition or declaration of THE 'property' in your code? Because it's nowhere to be find what is the 'property'.

This will be very helpful. Thanks.

Best Regard,

Yang Shen

Answer 8

User303363814 posted

So now you have two very, very slightly different pieces of code which work.  What problem are you trying to solve?  It is not clear.

Answer 9

User254389092 posted

Hi Yang shen, Trying to check not equals for this piece of code. Warning says: "change condition sothat it does not always evaluate to True"

// Invoke model property via expression
var property = expression.Compile().Invoke(model);
var propertyValue = !property.Equals(null) && property.ToString() != "0" ? property.ToString() : string.Empty;

the second part where I try to implement is:

// Invoke model property via expression
TProperty property = expression.Compile().Invoke(model);

output.AppendFormat(@"<div class='input-group '>
<input id='{0}' name='{0}' type='text' class='input-sm form-control{7}' value='{6}' title='{6}' />
<div class='input-group-btn'>
<button type='button' class='btn btn-default dropdown-toggle' {5} title='{1}' onclick='{3}(""{2}"",""{4}"")'>{1}
<span class='caret'></span>
</button>
</div>
</div>",
name,
buttonTitle,
recipientSelectorTitle,
buttonFunction,
urlHelper.Action("_RecipientSelector", "Recipient", new { Id = name, isPrivate = false }),
isReadOnly ? "disabled='disabled'" : "",
property.Equals(null) ? string.Empty : property.ToString(),
htmlHelper.ViewData.ModelState[name] != null && htmlHelper.ViewData.ModelState[name].Errors.Any() ?
" input-validation-error" : string.Empty);

I hope I gave you the right code. Please look into this 

Answer 10

User-719153870 posted

Hi Lucifer_deep,

I think we have made great progress.

But i will need also the definition of your model in expression.Compile().Invoke(model);.

Thanks.

Best Regard,

Yang Shen

Answer 11

User254389092 posted

Hi Yang Shen,

Please find the below code for model definition:

// Grab model from view
var model = (TModel)htmlHelper.ViewContext.ViewData.ModelMetadata.Model;

same model declaration was used to implement both the lines of code.

Luc

Answer 12

User-719153870 posted

Hi Luc,

Thank you so much, to be frank, i was not familiar with MVC html helper with Lambda expression.

But after several hours learning about this and also with the help of your code, i finally understand how this thing goes.

As for your question: 

I need to convert below line of code:

property == null ? string.Empty : property.ToString()

to use Equals to compare. Can it be done?

The answer is NO, you can't use object.equals() method to a null object.

For the method, it needs to work like xxx.Equals(null) instead of null.Equals(null).

When the propert is null, it will generate an error: Object reference not set to an instance of an object.

Lastly, if the property can be null in cases, you should use the property == null ? string.Empty : property.ToString().

Best Regard,

Yang Shen