Error converting data type varchar to numeric. Insert statement [duplicate]

Answer 1

This answer will address 2 issues

1. Protecting from SQL injection using Parameterized Queries
2. Converting to numeric values when required

1+2. (Please note that for expedience I did not code for all your parameters )

In your Code Behind:

using (SqlConnection conn = new SqlConnection(connStr))
{
   SqlCommand cmd = new SqlCommand();
   cmd.Connection = conn;
   cmd.CommandType = CommandType.Text;
   cmd.CommandText =  "INSERT INTO SalesActivity(Activity_ID, Date, Quatation_Number, Customer_ID, Product_ID, Quantity, valueGBR, valueEUR, Rate, weightedValue, Status_ID, estDecisionDate, PromisedDeliveryDate) values(@Activity,@Date, @param3 ,@param4,@param5,@param6,@param7,@param8,etc................... )";                }
   cmd.Parameters.AddWithValue("@Activity", Convert.ToInt32(txtActivity.Text));
}

....Do this for all your parameters (convert to Int32 as required)

Answer 2

In addition to @DaniDev answers, if you are not sure content is a valid int then safer option is

int val = 0;
Int32.TryParse( TextBox1.Text, out val );

This will provide you with some default value you can use. Int32.TryParse also returns a boolean value indicating whether it was able to parse or not, so you can even use it as the condition of an if statement.

See Int32.TryParse Method (String, Int32) for more detailed information.