Can not implicitly convert type 'string' to 'int'

You have to have the conversion done. As the data may not be valid you have to test to see if the conversion ran correctly. This is done with TryParse() .

protected void btnSave_Click(object sender, EventArgs e) {
    var aDay = new List<ProdutoAmortizacaoCreditoDias>();
    var ee = new ProdutoAmortizacaoCreditoDias();
    int dia;
    if (int.TryParse(this.txtDay.Text, out dia)) {
        ee.Dia = ee.Dia = dia;
    } else {
        //faz alguma coisa aqui porque deu erro na conversão.
    }
    if (ProcessingType != 1) {
        ee.FromProductID = this.hdnSourceID.Value;
        ee.ToProductID = this.hdnDestinationID.Value;
    }

I placed it on GitHub for future reference .

In C # 7 you can only do:

protected void btnSave_Click(object sender, EventArgs e) {
    var aDay = new List<ProdutoAmortizacaoCreditoDias>();
    var ee = new ProdutoAmortizacaoCreditoDias();
    if (int.TryParse(this.txtDay.Text, out var dia)) {
        ee.Dia = ee.Dia = dia;
    } else {
        //faz alguma coisa aqui porque deu erro na conversão.
    }
    if (ProcessingType != 1) {
        ee.FromProductID = this.hdnSourceID.Value;
        ee.ToProductID = this.hdnDestinationID.Value;
    }

I took the opportunity to give an organized one in the code.

Using c # just use the system's conversion class.

ee.Dia = System.Convert.ToInt32(this.lblDay.Text);

Detail: This conversion is if Dia is an int. If it is a long , use ToInt64 ; if it is a short, use ToInt16 .

More on link

Your Day property is probably of type Int, when you get this.lblDay.Text; the text by default is a string, so you can not assign a string to an Int type, you would have to convert before this:

ee.Dia =  Convert.ToInt32(this.lblDay.Text);

OR

ee.Dia =  int.Parse(this.lblDay.Text);

I believe it will work, but it's okay to validate that this field really is an integer value, you can do this with the int.TryParse example HERE