Loop through request.form and add data with db.Execute [duplicate]

I have a huge form and try to add the the data with only a few lines of code - without declaring all variables. What I'm acutally trying is this:

string all_fields="";
string all_params="";
string all_values="";
var i=0;

foreach(string key in Request.Form) {
    @Html.Raw(key + " " + Request.Form[key] + "\n")
    if(all_fields==""){
        all_fields="@"+i;
        all_params=key;
        all_values="\"" + Request.Form[key] + "\"";
    }else{
        all_fields+=", @"+i;
        all_params+=", "+key;
        all_values+=", \"" + Request.Form[key] + "\"";
    }
    i+=1;
}

var db = Database.Open("the_db");
var sql = "INSERT INTO the_table ("+all_fields+") Values("+all_params+")";  

db.Execute(sql, all_values); <--- Don't work

The content of the variables look like:

string all_fields="fistname,lastname";
string all_params="@0,@1";
string all_values="\"Jon\", \"Doe\"";

I didn't really expect db.Execute(sql, all_values); does work. it's a string but it needs to be an object: https://learn.microsoft.com/en-us/dotnet/api/webmatrix.data.database.execute?view=aspnet-webpages-3.2#WebMatrix_Data_Database_Execute_System_String_System_Object___

How can I create this object? Or anyone another solution? Ideas?

I know I could do it with SQL only. But then I would run into an security issue because of sql-injection.