Bind Multibinding Textbox using MVVM and Entity Framework

Question

I have 4 textboxes like, 3 take value and multiply it with corresponding bills
<TextBox Text="{Binding Path=Amount.Bill100,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox100Bills" Grid.Column="2" Grid.Row="1" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Amount.Bill50,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox50Bills" Grid.Column="2" Grid.Row="2" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Amount.Bill20,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox20Bills" Grid.Column="2" Grid.Row="3" Margin="7"></TextBox>
mLast text box adds the value and shows it like this
<TextBox Grid.Column="2" IsReadOnly="True" Grid.Row="7" Grid.ColumnSpan="2" Margin="7" Name="TotalAmount">
<TextBox.Text>
<MultiBinding Converter="{ikriv:MathConverter}" ConverterParameter="x+y+z" Mode="TwoWay">
<Binding Path="Text" ElementName="TextBox100Bills" />
<Binding Path="Text" ElementName="TextBox50Bills" />
<Binding Path="Text" ElementName="TextBox20Bills" />
</MultiBinding>
</TextBox.Text>
</TextBox>
How to bind this textbox to my entity properties the way I bind the 1st three.So that I will get all data in my entity object and save it in data base

Answer 1

Whatever you are doing in your converter class you might as well do in your view model class and then you bind the 4th TextBox to another property  (NewProperty in the sample code below) that gets set whenever the value of any the first three
properties is changed:
public class YourViewModel : System.ComponentModel.INotifyPropertyChanged
//+ the rest of your view model class...
public double Bill100
get { return Amount.Bill100; }
set
Amount.Bill100 = value;
OnPropertyChanged("Bill100");
this.NewProperty = this.Bill100 + this.Bill20 + this.Bill50;
public double Bill50
get { return Amount.Bill50; }
set
Amount.Bill50 = value;
OnPropertyChanged("Bill50");
this.NewProperty = this.Bill100 + this.Bill20 + this.Bill50;
public double Bill20
get { return Amount.Bill20; }
set
Amount.Bill20 = value;
OnPropertyChanged("Bill20");
this.NewProperty = this.Bill100 + this.Bill20 + this.Bill50;
private string _newProperty;
public string NewProperty
get { return _newProperty; }
set { _newProperty = value; OnPropertyChanged("NewProperty"); }
private void OnPropertyChanged(string property)
if (PropertyChanged != null)
PropertyChanged(this, new System.ComponentModel.PropertyChangedEventArgs(property));
public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
<TextBox Text="{Binding Path=Bill100,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox100Bills" Grid.Column="2" Grid.Row="1" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Bill50,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox50Bills" Grid.Column="2" Grid.Row="2" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Bill20,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="TextBox20Bills" Grid.Column="2" Grid.Row="3" Margin="7"></TextBox>
<TextBox Grid.Column="2" IsReadOnly="True" Grid.Row="7" Grid.ColumnSpan="2" Margin="7" Name="TotalAmount"
Text="{Binding NewProperty}"></TextBox>
You then save the value of the new property (NewProperty).
Note that the view model class must implement the INotifyPropertyChanged interface for the TextBoxes to get updated automatically when you set the value of the data bound source property:
https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.110).aspx.
You cannot get the value of a two-way binding from one property and then set the value of another property.
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