Question

C# comboBox下拉菜单取值做判断

在win form中做了comboBox1，comboBox2，button1，datagridview1，点击button1要求根据comboBox1，和comboBox2选择的值来确定datagridview的显示内容。但是comboBox1和comboBox2如果有一个不做选择的话，就只根据一个取值来确定datagridview的取值，我的部分代码如下：

string a, b;
a = comboBox1.SelectedValue.ToString();
b = comboBox2.SelectedValue.ToString();

string sql = "select* from content where logo=' “+a+” ', and subject=' “+b+” ';
sqlBase s = new sqlBase();
DataTable t = new DataTable();
t = s.ExeSQLdt(sql);
dataGridView1.DataSource = t;
dataGridView1.Columns[0].Width = 45;
dataGridView1.Columns[2].Width = 30;
dataGridView1.Columns[3].Width = 30;
dataGridView1.Columns[4].Width = 280;

以上代码执行的结果就是根据comboBox1和comboBox2的共同取值来确定从数据库中筛选的结果，如果有一个comboBox的下拉菜单不做选择，执行结果就会报错。怎么在上面加一个判断，比如只选择了comboBox1的值，comboBox2不选择，执行结果为
string sql = "select* from content where logo=' “+a+” ';
sqlBase s = new sqlBase();
DataTable t = new DataTable();
t = s.ExeSQLdt(sql);
dataGridView1.DataSource = t;
dataGridView1.Columns[0].Width = 45;
dataGridView1.Columns[2].Width = 30;
dataGridView1.Columns[3].Width = 30;
dataGridView1.Columns[4].Width = 280;

跪求结果，在线等待，谢谢

Answer 1

string a, b;
a = comboBox1.SelectedValue.ToString();
b = comboBox2.SelectedValue.ToString();

string sql = "select* from content where logo=' “+(String.IsNullOrEmpty(a)?"%":a)+” ', and subject=' “+(String.IsNullOrEmpty(b)?"%":b)+” ';
sqlBase s = new sqlBase();
DataTable t = new DataTable();
t = s.ExeSQLdt(sql);
dataGridView1.DataSource = t;
dataGridView1.Columns[0].Width = 45;
dataGridView1.Columns[2].Width = 30;
dataGridView1.Columns[3].Width = 30;
dataGridView1.Columns[4].Width = 280;

Answer 2

string sql = "select* from content where 1=1";
if (a!="")
{ sql += " and logo='“+a+” ' ";}
if (b!= "")
{ sql += " and subject='“+b+” ' ";}
...

追问

if(comboBox1.selectedItem!=null) 这样判断，不要再声明变量a，b会简便一些，不过仍然非常感谢

Answer 3

HAL9000PLUS 的