Difference between @bind and @bind-value

Question

What is the difference of using @bind and @bind-value?

I made this simple example, and testing it in the browser, I didn't see any difference.

<p>@@bind @increment1</p>

<input 
    type="text"
    @bind="@increment1"
/>

<p>@@bind-value @increment2</p>
<input 
    type="text"
    @bind-value="@increment2"
/>

@code {
    string increment1;
    string increment2;
}

Answer 1

Short Version

@bind is an override of @bind-value with the event set to "onchange".

These two commands are equivalent:

 ... @bind-value="userName" @bind-value:event="onchange" ...
 ... @bind="userName" ...

Long Version

The @bind attribute accomplishes two separate (but related) tasks:

1. Binds an expression to the value of the <Input... component
2. Binds a delegate that will trigger the component's ValueChanged property

Both the expression and the delegate are required. An implementation of @bind-Value looks like this:

 ... @bind-value="userName" @bind-value:event="onchange" ...

We are setting both the expression (="userName") and the delegate (="onchange").

The "easier" @bind= is just an override with the delegate preset to "onchange". So these two commands are functionally the same:

 ... @bind-value="userName" @bind-value:event="onchange" ...
 ... @bind="userName" ...

A greatly simplified analogy using overriding methods:

public void bind-value(string value, string event)
{..}

public void bind(string value)
{
  bind-value(value, "onchange");
}

A couple of common use-cases for using the full @bind-value version are

1. updating the UI as the user types
2. validating an email address as the user types

Remember, the onchange event will only trigger PropertyChanged when the component loses focus. Instead, we want PropertyChanged to be triggered by the oninput event:

... @bind-value="H1Content" @bind-value:event="oninput" ...
... @bind-value="email" @bind-value:event="oninput" ...

Answer 2

EDITED because @dragon-warrior and @somedotnetguy coments.

ASP.NET >= Core 3.1

@bind-value:event is not more required for events. Just use @bind:event.

ASP.NET Core 3.0

Quoting ASP.NET Core 3.0 Component Binding (currently unavailable) docs:

Data binding to both components and DOM elements is accomplished with the @bind attribute. (...) Using @bind with a CurrentValue property (<input @bind="CurrentValue" />) is essentially equivalent to the following:

<input value="@CurrentValue"
       @onchange="@((ChangeEventArgs __e) => CurrentValue = __e.Value)" />

In addition to handling onchange events with @bind syntax, a property or field can be bound using other events by specifying an @bind-value attribute with an event parameter (@bind-value:event). ( onchange, oninput )

Summarizing

If you want to reset binded value on each input change (instead to set all changes at once on lost input focus) you should to use @bind-value and oninput on @bind-value:event:

<input @bind-value="CurrentValue" 
       @bind-value:event="oninput" />

If you try to use @bind-value:event without @bind-value (using just @bind ) a pre-compiling error is raised:

error RZ10004: Could not find the non-parameterized bind attribute that corresponds to the attribute 'bind-value:event'

But the XXX.razor.g.cs generated file is the same for @bind and @bind-value.

Answer 3

There is not any significant deference between these two. The you can use @bind-value and @bind-value:event or you can use @bind and @bind:event pairs arbitrary.