如何解决“指定的演员表无效”。 错误

Question

I have simple project, I dynamically execute a string value with C# compiler .

follow code:

private void btntest_Click(object sender, EventArgs e)
        {
            string code = @"
using System;
using System.Text;
using System.Data;
using System.Reflection;
using System.ComponentModel;

namespace myNameSpace
{
    public class DynamicClass
    {
        public bool myFunction(DataRow dr)
        {
            if (dr == null)
                return false;

            if (dr.Table.Columns.Contains(""Col1"")  && (bool)dr[""Col1""] == true)
                return true;
            else
                return false;
        }
    }
}
";

            Assembly asm = BuildAssembly(code);
            object instance = null;
            Type type = null;
            instance = asm.CreateInstance("myNameSpace.DynamicClass");
            type = instance.GetType();
            DataTable dt = new DataTable();
            dt.Columns.Add("Col1");
            DataRow dr = dt.NewRow();
            dr[0] = true;

            MethodInfo method = type.GetMethod("myFunction");
            object returnVal = method.Invoke(instance, new object[] { dr });
            txtError.Text = returnVal.ToString();

        }

        Assembly BuildAssembly(string code)
        {
            CodeDomProvider codeProvider = CodeDomProvider.CreateProvider("CSharp");
            CompilerParameters compilerparams = new CompilerParameters();
            compilerparams.GenerateExecutable = false;
            compilerparams.GenerateInMemory = false; 

            compilerparams.ReferencedAssemblies.Add("mscorlib.dll");
            compilerparams.ReferencedAssemblies.Add("System.dll");
            compilerparams.ReferencedAssemblies.Add("System.Data.dll");
            compilerparams.ReferencedAssemblies.Add("System.Xml.dll");

            CompilerResults results = codeProvider.CompileAssemblyFromSource(compilerparams, code);
            if (results.Errors.HasErrors)
            {
                StringBuilder errors = new StringBuilder("Compiler Errors :\r\n");
                foreach (CompilerError error in results.Errors)
                {
                    errors.AppendFormat("Line {0},{1}\t: {2}\n",
                           error.Line, error.Column, error.ErrorText);
                }
                throw new Exception(errors.ToString());
            }
            else
            {
                return results.CompiledAssembly;
            }
        }

when I run the project,I get the following error:

Exception has been thrown by the target of an invocation.

Inner Exception

InvalidCastException: Specified cast is not valid.

if I remove the (bool)dr[""Col1""] == true section, it's working well.

what assembly or namespace should be added?

How to fix this error?

Any help is much appreciated.

Answer 1

The problem is this line:

dt.Columns.Add("Col1");

If you do not specify a type, it will default to string . This means that the value of the cell is getting converted to a string True . So you should be doing this:

dt.Columns.Add("Col1", typeof(bool));

Answer 2

As you have already pointed out, this is the line causing the issue:

if (dr.Table.Columns.Contains("Col1")  && (bool)dr["Col1"] == true)

The explicit cast of dr["Col1"] to a bool is failing because the value is not a boolean . The explicit cast fails.

Assuming that dr["Col1"] should actually contain a bool but may not always, you could verify it's type before converting using syntax like this:

dr["Col1"] is bool myValue && myValue == true

The above code verified that dr["Col1"] is a bool and then defined myValue as that bool value.

The code can become even cleaner by removing myValue == true and just having myValue (as it will be evaluated as true or false ). In your quoted code:

if (dr.Table.Columns.Contains(""Col1"") && dr[""Col1""] is bool myValue && myValue)

If dr["Col1"] does not actually contain a bool but actually contains an int or something else as an indicator of the value being true you will need to implement different logic.