Why Can't I Use System.Io.File Methods in an MVC Controller

Why can't I use System.IO.File methods in an MVC controller?

You should prefix it with a namespace:

if (System.IO.File.Exists(picPath))
{ 
    //Other code
}

The reason for that is because you are writing this code inside a controller action which already defines a File method on the Controller class.

How to access the static System.IO.File class from an MVC Controller?

In place of File.Delete(path) which, within the context of a controller will use the controllers version of the method; you need to use System.IO.File.Delete() exactly like that to access the system input/output version (which is the one you need). If you don't specify System.IO.File. before delete it will default to the controllers version even if you have a using statement for System.IO at the top of your file.

How Can I Use File class in an MVC Controller in ASP.Net?

You can use the full namespace to explicitly point at the correct File class:

using (var fileStream = System.IO.File.Create(path))

File is a Method which is not valid in the given context

You probably have this method inside MVC controller in which File method exists. Add in your code System.IO.File instead of File

Unable to use File.WriteAllBytes() in asp.net MVC, cannot choose method from method group did you intend to invoke the method?

It sounds like you have a method that's visible to you (in the same class, say) called File. This will be chosen in preference to System.IO.File. So you need to fully qualify the name:

System.IO.File.WriteAllBytes(tempfilenameandlocation, thePDFLetter);

(In fact, enough clues were in the question. It's Controller.File that's in scope)

C# to ASP.NET MVC FileStream Crossover

You'll have to add the fully-qualified name if you want to use the File class in System.IO (http://msdn.microsoft.com/en-us/library/system.io.file.aspx). So something like this should work:

using (FileStream stream = System.IO.File.OpenRead("database.dat")){
    database = (List)formatter.Deserialize(stream);
}

ControllerBase.File(byte[], string)' is a method, which is not valid in the given context (CS0119) - in method

1) It is ok to put uploading into separate method, it could also be put into a separate class for handling blob operations

2) File is the name of one of the controllers methods, if you want to reference the File class from System.IO namespace, you need to fully qualify the name

FileStream uploadFileStream = System.IO.File.OpenRead(localFilePath);

To the other compile error, you need to return something from the UploadToBlob method, now it does not return anything from the try block

3) File type validation can be put into the controller action method

4) it depends on what you plan to do with the text and how are you going to use it. Would it be a new action of the controller (a new API endpoint)?

5) you could create a new API endpoint for downloading files

UPDATE:

For word replacement you could use a similar method:

private Stream FindMostFrequentWordAndReplaceIt(Stream inputStream)
{
    using (var sr = new StreamReader(inputStream, Encoding.UTF8)) // what is the encoding of the text? 
    {
        var allText = sr.ReadToEnd(); // read all text into memory
        // TODO: Find most frequent word in allText
        // replace the word allText.Replace(oldValue, newValue, stringComparison)
        var resultText = allText.Replace(...);

        var result = new MemoryStream();
        using (var sw = new StreamWriter(result))
        {
            sw.Write(resultText);
        }
        result.Position = 0;
        return result;
    }
}

it would be used in your Post method this way:

using (var stream = formFile.OpenReadStream())
{
    var streamWithReplacement = FindMostFrequentWordAndReplaceIt(stream);

    // Upload the replaced text:
    (uploadSuccess, uploadedUri) = await UploadToBlob(formFile.FileName, null, streamWithReplacement);

}

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