Issue with C# lambda return System.Linq.Expressions.Expression

Question

I have a big problem with assigning the following code and I don't know how to solve it and I want to use lambda operators.

 System.Linq.Expressions.Expression expression = null;
 if (typeof(ICustomTypeDescriptor).IsAssignableFrom(sourceType))
 {
            System.Linq.Expressions.Expression expression2 = System.Linq.Expressions.Expression.Convert(parameterExpression, typeof(ICustomTypeDescriptor));
            expression = (ICustomTypeDescriptor t, object o) =>
            {
                t.GetProperties()[propertyName].GetValue(o)
                }
            ;
            return System.Linq.Expressions.Expression.Invoke(expression, new System.Linq.Expressions.Expression[]
            {
                expression2,
                parameterExpression
            });
        }

The problem is in this part enter image description here

Can it be declared in any other way?

ERROR

Unable to convert lambda expression element to "Expression" type because it is not a delegate type

Answer 1

You are using multilines lambda expression, which is ok. But you need to end each line with ";".

I think this would work but without the whole code it will be hard to be sure:

System.Linq.Expressions.Expression expression = null;
if (typeof(ICustomTypeDescriptor).IsAssignableFrom(sourceType))
{
    System.Linq.Expressions.Expression expression2 = System.Linq.Expressions.Expression.Convert(parameterExpression, typeof(ICustomTypeDescriptor));
    expression = (ICustomTypeDescriptor t, object o) =>
    {
        t.GetProperties()[propertyName].GetValue(o);
    };
    return System.Linq.Expressions.Expression.Invoke(expression, new System.Linq.Expressions.Expression[]
    {
        expression2,
        parameterExpression
    });
}

You can also go for singleline lambda:

System.Linq.Expressions.Expression expression = null;
if (typeof(ICustomTypeDescriptor).IsAssignableFrom(sourceType))
{
    System.Linq.Expressions.Expression expression2 = System.Linq.Expressions.Expression.Convert(parameterExpression, typeof(ICustomTypeDescriptor));
    expression = (ICustomTypeDescriptor t, object o) => t.GetProperties()[propertyName].GetValue(o);
    return System.Linq.Expressions.Expression.Invoke(expression, new System.Linq.Expressions.Expression[]
    {
        expression2,
        parameterExpression
    });
}

Please care, I haven't tested the code to check if I'm not wrong.

Answer 2

Instead of this line,

System.Linq.Expressions.Expression expression = null;

Create a delegate and assign a variable to your expression.

delegate    System.Linq.Expressions.Expression Myexpression(ICustomTypeDescriptor t, object o);

Myexpression expression = (ICustomTypeDescriptor t, object o) => t.GetProperties()[propertyName].GetValue(o);

Answer 3

Thanks guys, I'm sorry for not writing for so long, I found a solution and changed the code a bit, I tried with every proposal but unfortunately it threw the same error. Maybe my solution for someone will be helpful in the future:)

  private static LambdaExpression GetLambdaWithPropertyNullCheck(
      IEnumerable source,
      string propertyName,
      ParameterExpression parameterExpression,
      Type sourceType)
    {
        string[] strArray = propertyName.Split('.');
        LambdaExpression lambdaExpression;
        if (strArray.GetLength(0) > 1)
        {
            System.Linq.Expressions.Expression expression1 = parameterExpression.GetValueExpression(propertyName, sourceType);
            if (expression1.Type != typeof(object))
                expression1 = (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Convert(expression1, typeof(object));
            System.Linq.Expressions.Expression expression2 = (System.Linq.Expressions.Expression)null;
            string propertyName1 = string.Empty;
            int length = strArray.GetLength(0);
            for (int index = 0; index < length; ++index)
            {
                if (index == 0)
                {
                    expression2 = (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Equal(parameterExpression.GetValueExpression(strArray[index], sourceType), (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Constant((object)null));
                    propertyName1 = strArray[index];
                }
                else if (index < length - 1)
                {
                    propertyName1 = propertyName1 + (object)'.' + strArray[index];
                    expression2 = (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.OrElse(expression2, (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Equal(parameterExpression.GetValueExpression(propertyName1, sourceType), (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Constant((object)null)));
                }
            }
            lambdaExpression = System.Linq.Expressions.Expression.Lambda((System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Condition(expression2, (System.Linq.Expressions.Expression)System.Linq.Expressions.Expression.Constant((object)null), expression1, typeof(object)), parameterExpression);
        }
        else
            lambdaExpression = System.Linq.Expressions.Expression.Lambda(parameterExpression.GetValueExpression(propertyName, sourceType), parameterExpression);
        return lambdaExpression;
    }