Microsoft this week makes its pitch to a constituency that will go a long way toward determining the fate of the upcoming version of its flagship Windows software: developers.

Thousands of external developers, Microsoft says, will converge in San Francisco this week for the company's annual Build conference. The event, held Wednesday through Friday, is designed to give developers updates about the company's direction and help them craft programs for Microsoft's ecosystem of platforms.

Developers who write the code that makes programs from word-processing software and business-expense tools helped make Windows the dominant computing platform for much of the past two decades.

As Microsoft gets set to launch Windows 10 sometime this summer, however, the company faces a more complicated world. In addition to Windows, developers can build programs for the hundreds of millions of users of Apple's phones, tablets and laptops, as well as those plugged in to Google's ubiquitous free-to-use Android operating system.

"The collective view, at least from investors, is that they are way behind," said Karl Keirstead, a software analyst with Deutsche Bank. "Developers are not falling over themselves looking to build apps on Windows. They need to tell a very good story at Build to try to motivate them to do that."

Microsoft is hampered by its small slice of the global market for mobile devices. The smartphone-tailored version of Windows powered less than 3 percent of the devices sold last year, according to researcher IDC. The company occupies a similarly small slice of the tablet market.

Microsoft, which estimates that about 1.5 billion people use Windows, thinks Windows 10 will help remedy that. The company has said that it will offer the vast majority of individual Windows users free upgrades to Windows 10, and keep the operating system current for the lifetime of the device.

The goal is a virtuous cycle: Developers are motivated to reach the mass of people using Windows 10 across devices, and the proliferation of must-have programs in turn draws users to Windows smartphones and tablets.

The company has said the operating system will smooth the conversion of a desktop application to smartphone editions and the other way around.

"What they're trying to do is get this one platform running everywhere, and give developers an app that they can write once and optimize for other platforms," said Wes Miller, an analyst with Directions on Microsoft.

Microsoft hasn't disclosed its roster of speakers for this week's event, but analysts expect to hear from senior executives such as Chief Executive Satya Nadella and Windows leader Terry Myerson.

Nadella said during an earnings conference call last week that the company would share more details about Windows 10 and use of the company's Office productivity suite as a tool for developers.

In addition to Windows and Office, Microsoft also hopes to get developers hooked on the company's growing array of Web-based platforms. Microsoft is spending billions of dollars to build servers it rents to businesses for everything from data storage to processing power that can act as the plumbing for Web-based applications and tasks.

The company, which had a reputation during its younger years for pushing users and developers to its own platforms, has recently used its online offerings to showcase its more open side. Microsoft's Azure cloud infrastructure supports both Windows Server and its open-source rival, Linux. Office 365 links with Microsoft's customer relationship software rival Salesforce.com, as well as data-storage company Dropbox.

"They're doing all the right things," Deutsche Bank's Keirstead said. "It's just that, at least from a mobile standpoint, they have such a mountain to climb."

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