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patterncsharpMinor

Improvements to a ViewModelBase

Submitted by: @import:stackexchange-codereview··
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improvementsviewmodelbasestackoverflow

Problem

Every time I go to write an MVVM application (say every 4-6 months) I rewrite my ViewModelBase class. This is for a range of reasons but let's say either I don't have access to prior code, the previous code was built for a client and isn't my code, or I'm working on someone else's machine and still don't have the code.

So I rewrite the basic INotifyPropertyChanged implementation, and I think it's a little different each time. Here's what I've come up with today:

public class ViewModelBase : INotifyPropertyChanged
{
    protected void OnPropertyChanged(Expression> expression)
    {
        var property = (MemberExpression)expression.Body;
        this.OnPropertyChanged(property.Member.Name);
    }

    protected void OnPropertyChanged(string name)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}


I would like to get a nice implementation that is simple and small, and any improvements to this (what other functionality is required in a ViewModelBase?) would be great.

Solution

I've used the below for a while:

public abstract class PropertyChangedBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName)
    {
        var propertyChanged = this.PropertyChanged;

        if (propertyChanged != null)
        {
            propertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    protected bool SetProperty(ref T backingField, T Value, Expression> propertyExpression)
    {
        var changed = !EqualityComparer.Default.Equals(backingField, Value);

        if (changed)
        {
            backingField = Value;
            this.RaisePropertyChanged(ExtractPropertyName(propertyExpression));
        }

        return changed;
    }

    private static string ExtractPropertyName(Expression> propertyExpression)
    {
        var memberExp = propertyExpression.Body as MemberExpression;

        if (memberExp == null)
        {
            throw new ArgumentException("Expression must be a MemberExpression.", "propertyExpression");
        }

        return memberExp.Member.Name;
    }
}


I can write my properties like this:

private int id;

    public int Id
    {
        get
        {
            return this.id;
        }

        set
        {
            this.SetProperty(ref this.id, value, () => this.Id);
        }
    }

Code Snippets

public abstract class PropertyChangedBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName)
    {
        var propertyChanged = this.PropertyChanged;

        if (propertyChanged != null)
        {
            propertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    protected bool SetProperty<T>(ref T backingField, T Value, Expression<Func<T>> propertyExpression)
    {
        var changed = !EqualityComparer<T>.Default.Equals(backingField, Value);

        if (changed)
        {
            backingField = Value;
            this.RaisePropertyChanged(ExtractPropertyName(propertyExpression));
        }

        return changed;
    }

    private static string ExtractPropertyName<T>(Expression<Func<T>> propertyExpression)
    {
        var memberExp = propertyExpression.Body as MemberExpression;

        if (memberExp == null)
        {
            throw new ArgumentException("Expression must be a MemberExpression.", "propertyExpression");
        }

        return memberExp.Member.Name;
    }
}
private int id;

    public int Id
    {
        get
        {
            return this.id;
        }

        set
        {
            this.SetProperty(ref this.id, value, () => this.Id);
        }
    }

Context

StackExchange Code Review Q#13823, answer score: 7

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