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This topic describes how to effectively use and create Freezable objects, which provide special features that can help improve application performance. Examples of freezable objects include brushes, pens, transformations, geometries, and animations.

What Is a Freezable?

A Freezable is a special type of object that has two states: unfrozen and frozen. When unfrozen, a Freezable appears to behave like any other object. When frozen, a Freezable can no longer be modified.

A Freezable provides a Changed event to notify observers of any modifications to the object. Freezing a Freezable can improve its performance, because it no longer needs to spend resources on change notifications. A frozen Freezable can also be shared across threads, while an unfrozen Freezable cannot.

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Although the Freezable class has many applications, most Freezable objects in Windows Presentation Foundation (WPF) are related to the graphics sub-system.

The Freezable class makes it easier to use certain graphics system objects and can help improve application performance. Examples of types that inherit from Freezable include the Brush, Transform, and Geometry classes. Because they contain unmanaged resources, the system must monitor these objects for modifications, and then update their corresponding unmanaged resources when there is a change to the original object. Even if you don't actually modify a graphics system object, the system must still spend some of its resources monitoring the object, in case you do change it.

For example, suppose you create a SolidColorBrush brush and use it to paint the background of a button.

When the button is rendered, the WPF graphics sub-system uses the information you provided to paint a group of pixels to create the appearance of a button. Although you used a solid color brush to describe how the button should be painted, your solid color brush doesn't actually do the painting. The graphics system generates fast, low-level objects for the button and the brush, and it is those objects that actually appear on the screen.

If you were to modify the brush, those low-level objects would have to be regenerated. The freezable class is what gives a brush the ability to find its corresponding generated, low-level objects and to update them when it changes. When this ability is enabled, the brush is said to be 'unfrozen.'

A freezable's Freeze method enables you to disable this self-updating ability. You can use this method to make the brush become 'frozen,' or unmodifiable.

Note

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Not every Freezable object can be frozen. To avoid throwing an InvalidOperationException, check the value of the Freezable object's CanFreeze property to determine whether it can be frozen before attempting to freeze it.

When you no longer need to modify a freezable, freezing it provides performance benefits. If you were to freeze the brush in this example, the graphics system would no longer need to monitor it for changes. The graphics system can also make other optimizations, because it knows the brush won't change.

Note

For convenience, freezable objects remain unfrozen unless you explicitly freeze them.

Using Freezables

Using an unfrozen freezable is like using any other type of object. In the following example, the color of a SolidColorBrush is changed from yellow to red after it's used to paint the background of a button. The graphics system works behind the scenes to automatically change the button from yellow to red the next time the screen is refreshed.

Freezing a Freezable

To make a Freezable unmodifiable, you call its Freeze method. When you freeze an object that contains freezable objects, those objects are frozen as well. For example, if you freeze a PathGeometry, the figures and segments it contains would be frozen too.

A Freezable can't be frozen if any of the following are true:

  • It has animated or data bound properties.

  • It has properties set by a dynamic resource. (See the XAML Resources for more information about dynamic resources.)

  • It contains Freezable sub-objects that can't be frozen.

If these conditions are false, and you don't intend to modify the Freezable, then you should freeze it to gain the performance benefits described earlier.

Once you call a freezable's Freeze method, it can no longer be modified. Attempting to modify a frozen object causes an InvalidOperationException to be thrown. The following code throws an exception, because we attempt to modify the brush after it's been frozen.

To avoid throwing this exception, you can use the IsFrozen method to determine whether a Freezable is frozen.

In the preceding code example, a modifiable copy was made of a frozen object using the Clone method. The next section discusses cloning in more detail.

Note

Because a frozen freezable cannot be animated, the animation system will automatically create modifiable clones of frozen Freezable objects when you try to animate them with a Storyboard. To eliminate the performance overhead caused by cloning, leave an object unfrozen if you intend to animate it. For more information about animating with storyboards, see the Storyboards Overview.

Freezing from Markup

To freeze a Freezable object declared in markup, you use the PresentationOptions:Freeze attribute. In the following example, a SolidColorBrush is declared as a page resource and frozen. It is then used to set the background of a button.

To use the Freeze attribute, you must map to the presentation options namespace: http://schemas.microsoft.com/winfx/2006/xaml/presentation/options. PresentationOptions is the recommended prefix for mapping this namespace:

Because not all XAML readers recognize this attribute, it's recommended that you use the mc:Ignorable Attribute to mark the Presentation:Freeze attribute as ignorable:

For more information, see the mc:Ignorable Attribute page.

'Unfreezing' a Freezable

Once frozen, a Freezable can never be modified or unfrozen; however, you can create an unfrozen clone using the Clone or CloneCurrentValue method.

In the following example, the button's background is set with a brush and that brush is then frozen. An unfrozen copy is made of the brush using the Clone method. The clone is modified and used to change the button's background from yellow to red.

Note

Regardless of which clone method you use, animations are never copied to the new Freezable.

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The Clone and CloneCurrentValue methods produce deep copies of the freezable. If the freezable contains other frozen freezable objects, they are also cloned and made modifiable. For example, if you clone a frozen PathGeometry to make it modifiable, the figures and segments it contains are also copied and made modifiable.

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Creating Your Own Freezable Class

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A class that derives from Freezable gains the following features.

  • Special states: a read-only (frozen) and a writable state.

  • Thread safety: a frozen Freezable can be shared across threads.

  • Detailed change notification: Unlike other DependencyObjects, Freezable objects provide change notifications when sub-property values change.

  • Easy cloning: the Freezable class has already implemented several methods that produce deep clones.

A Freezable is a type of DependencyObject, and therefore uses the dependency property system. Your class properties don't have to be dependency properties, but using dependency properties will reduce the amount of code you have to write, because the Freezable class was designed with dependency properties in mind. For more information about the dependency property system, see the Dependency Properties Overview.

Every Freezable subclass must override the CreateInstanceCore method. If your class uses dependency properties for all its data, you're finished.

If your class contains non-dependency property data members, you must also override the following methods:

You must also observe the following rules for accessing and writing to data members that are not dependency properties:

  • At the beginning of any API that reads non-dependency property data members, call the ReadPreamble method.

  • At the beginning of any API that writes non-dependency property data members, call the WritePreamble method. (Once you've called WritePreamble in an API, you don't need to make an additional call to ReadPreamble if you also read non-dependency property data members.)

  • Call the WritePostscript method before exiting methods that write to non-dependency property data members.

If your class contains non-dependency-property data members that are DependencyObject objects, you must also call the OnFreezablePropertyChanged method each time you change one of their values, even if you're setting the member to null.

Note

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Super player 2010 chinese film. It's very important that you begin each Freezable method you override with a call to the base implementation.

For an example of a custom Freezable class, see the Custom Animation Sample.

See also

Some of us like to try DIY solutions. If we can do it ourselves, why would we pay someone else to do it, right?

If you have a spot, like a wart or a mole, that needs to be removed, you may have considered freezing it at home rather than asking your dermatologist about it. Maybe you think an over-the-counter option is good enough, or you assume the dermatologist is too expensive. Either way, before you rush to the nearest drugstore to buy a freeze spray, make sure you know what to expect.

Will Freezing Remove the Spots On My Skin?

Not all spots can be frozen, but warts and seborrheic keratosis (a type of brown mole) respond well to removal by freezing. However, there’s a big difference between at-home removal and dermatologist removal. Dermatologists not only tailor their approach to your spot, but they have access to a stronger freezing product. This is especially important for a wart, because it needs to be frozen deeply, requiring a lot of energy and a strong product to remove it effectively.

Freezing Warts at Home

Freezing at home relies on a butane spray. There’s a little liquid butane in the can that turns to gas as it’s sprayed. The coldest temperature of that gas is about 100 degrees below zero. Usually, people hold it a few inches away from the spot, so it’s about negative 50 degrees when it reaches the skin. Yes, that’s extremely cold, but it’s not cold enough to freeze deeply. That means they have to spray the butane longer, which really hurts. So, in the hands of most people, butane spray doesn’t effectively remove warts.

Freezing Warts at the Dermatologist’s Office

The dermatologist uses liquid nitrogen, either spraying it on or dropping it on with a Q-tip. Since it’s 320 degrees below zero, it freezes the skin rapidly at a much deeper level than butane. Essentially, it damages the spot quickly and severely so we can remove it. But that also means people have to be very careful with the product.

Why You Should See A Dermatologist For Wart Removal

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With a dermatologist, you get expertise and a more effective product to treat your spots. To remove these blemishes, we’re creating a superficial frostbite. That can’t be done haphazardly. Dermatologists are trained to use liquid nitrogen as a tool in effectively removing spots. But, in untrained hands, damage from this product can quickly become extensive.

When you pay for your dermatologist to freeze a trouble spot, you get an educated professional using the right tool and the right amount of destruction to remove the spot effectively. Someone unfamiliar with using a freezing instrument probably won’t deliver enough damage to the tissue to remove the spot, or they’ll use too much and cause extensive damage.

Are Freezing Treatments Covered by Insurance?

“Cryotherapy” is the technical name for any freezing treatment. It’s a procedure well recognized in medicine as an effective tool to treat many different issues, not just skin conditions.

You’ve probably heard about cryotherapy for athletes. These treatments can help with a variety of muscle ailments as patients stand in a tank filled with liquid nitrogen vapor. But that’s not the kind of cryotherapy we do.

Cryotherapy at our office is considered a procedure in insurance terms. That means there’s a charge for cryotherapy outside of your office visit copay that counts towards your deductible.

Here’s how procedures work with most insurance companies. Typically, you have a deductible. That’s the amount you pay before your insurance starts paying. So let’s say your deductible is $1,000. You’ll pay all expenses up to that amount. Then, after you have paid $1,000, your insurance will start paying for most of your costs. They may pay for 80% of the bill, and you pay the remaining 20%.

Think of the payment in two parts. First, you pay your copay — the payment for the doctor to see you, evaluate your condition, and make a diagnosis. Second, you pay for the procedure. So, if you’re having a wart frozen, you’ll pay for cryotherapy.

Keep in mind, your insurance company sets the rate for your copay, deductible, and each specific treatment. Usually, the copay is slightly higher for a specialist. Those rates aren’t set by the doctor — they’re determined by your insurance plan.

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The insurance company also determines what amount is a fair payment for each specific treatment. So they may set the rate for a wart removal at $100. If you haven’t met your deductible, you’ll pay the $100. If you have met your deductible, the insurance company will pay their percentage as explained in your plan.

Do Any At-Home Treatments Work?

There are good over-the-counter medicines to treat warts, but they require a lot of persistence and research. You must understand how to use the specific medicine, which can be a complex process. Then you’ll need to stick with the treatment until the wart is gone.

For example, there’s an over-the-counter wart remover liquid that you paint on warts. It’s a high concentration of salicylic acid that causes the skin to slough off. People often get frustrated with the product because they don’t know how to remove the white, dead skin cells in between treatments. So they keep applying the product but don’t know why it doesn’t work. These treatments can work if you do them correctly, but it may take a long time to see the full results.

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A dermatologist is a one-stop shop for an issue like wart and mole removal. Liquid nitrogen is so effective, we can resolve the issue quickly and let patients move on with their lives.

Dr. R. Todd Plott is a board-certified dermatologist in Coppell, Keller, and Saginaw, TX. His specialization and professional interests include treating patients suffering with acne, identifying and solving complex skin conditions such as psoriasis, rosacea, atopic dermatitis, and identifying and treating all types of skin cancers. In his spare time, Dr. Plott enjoys cycling, traveling with his wife, and spending time with his children and new grandson.

Learn more about Dr. Plott.